The first injustice Socio-economic inequalities in birth outcome

The first injustice
Socio-economic inequalities
in birth outcome
by
Marit Dahlén Gisselmann
Health Equity Studies No 8
Centre for Health Equity Studies (CHESS)
Stockholm University/Karolinska Institutet 2007
© Marit Dahlén Gisselmann 2007
Cover photography: Pieter Bregman
Graphic design: Klickadit Design AB, MariaPia Gistedt
Printed by Elanders Gotab, Stockholm 2007
ISSN 1651-5390
ISBN 978-91-7155-429-1
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Contents
Abstract ...........................................................................................................6
Svensk sammanfattning..................................................................................7
List of publications...........................................................................................8
Introduction .....................................................................................................9
Outline of thesis............................................................................................................. 11
What are socio-economic inequalities in birth outcome?..............................12
Birth outcomes in this thesis .................................................................................... 12
Causes of disadvantageous birth outcomes differ................................................... 14
Birth outcomes as health indicators......................................................................... 15
Socio-economic indicators ............................................................................................ 16
Measuring social class for women........................................................................... 17
Previous Nordic research..............................................................................20
Mortality.................................................................................................................... 22
Birthweight ............................................................................................................... 23
Gestational age and foetal growth ........................................................................... 24
Explanatory approaches................................................................................................ 24
Demographic factors................................................................................................ 25
Mapping out an explanatory framework........................................................27
The link between maternal and infant health ................................................................ 27
The framework............................................................................................................... 28
Ecological factors providing a context ..................................................................... 30
Maternal socialisation, social selection (1) .............................................................. 31
Health in early maternal life: socio-economic inequalities (2) ................................. 32
Health in early maternal life: biological imprinting (3) .............................................. 32
Health in early maternal life: accumulated health (3) .............................................. 33
Socio-economic status in early maternal life (4)...................................................... 33
Health selection from childhood to adult socio-economic status (5) ............................. 34
Health selection in adult life (6)................................................................................ 34
Adult socio-economic status and adult health: living conditions (7).............................. 35
Adult socio-economic status and adult health: lifestyle and behaviours (7)............ 36
Direct references to maternal health indicators (8).................................................. 37
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Socio-economic factors after birth (9)...................................................................... 38
Access to care.......................................................................................................... 39
Stress – an important link? ...................................................................................... 39
Transfer of health between generations........................................................................ 40
Studies ..........................................................................................................42
Specific aims ................................................................................................................. 42
Data ............................................................................................................................... 42
MEGA Database, Studies I-III.................................................................................. 42
Social Mobility Database, Study IV.......................................................................... 44
Completeness and quality of registers..................................................................... 44
Results and interpretations............................................................................................ 44
Study I: Maternal education ..................................................................................... 44
Study II: Maternal childhood and adult social class................................................. 45
Study III: Maternal working conditions and infant mortality ..................................... 46
Study IV: Maternal working conditions and birth outcome....................................... 46
Discussion.....................................................................................................48
Socio-economic status, individuals and households..................................................... 49
Confounders and mediators .......................................................................................... 50
The relevance of the low birthweight paradox............................................................... 51
The dynamic character of health inequalities................................................................ 52
Some conclusions – relevance of the results................................................54
Acknowledgements .......................................................................................55
References....................................................................................................57
5
Abstract
Adverse birth outcomes like preterm birth and infant mortality are unevenly
distributed across socio-economic groups. Risks are usually lowest in groups
with high socio-economic status and increase with decreasing status.
The general aim of this thesis was to contribute to the understanding of
the relation between socio-economic status and birth outcomes, focussing on
maternal education and class, studying a range of birth outcomes. More specific aims were to investigate the relation between maternal education and
infant health, to study the combined influence of maternal childhood and
adult social class on inequalities in infant health and to explore the contribution of maternal working conditions to class inequalities in birth outcomes.
The studies are population based, focussing on singletons births 1973-1990.
During the period under study, educational differences in birth outcomes
increased, especially between those with the lowest and highest education.
The low birth weight paradox emerged, suggesting that the distribution of
determinants for low birthweight infants differs for these groups.
Further, an independent association was found between maternal childhood social class and low birthweight and neonatal mortality, but not for
postneonatal mortality. Since this was found for the two outcomes closest to
birth, this indicates that the association is mediated through the maternal
body.
Finally, there is a contribution of maternal working conditions to class
inequalities in birth outcome. Lower job control, higher job hazards and
higher physical demands were all to some degree related to increased risk of
the following adverse birth outcomes: infant mortality, low birthweight, very
low birthweight, foetal growth, preterm birth, very and extremely preterm
birth. Working conditions demonstrated disparate associations with the birth
outcomes, indicating a high complexity in these relationships.
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Svensk sammanfattning
Risker för mer ofördelaktiga hälsoutfall, såsom för tidig födsel eller spädbarnsdödlighet, är inte jämt fördelade över socio-ekonomiska grupper ens i ett relativt jämlikt välfärdssamhälle såsom Sverige. Vanligen finns de lägsta riskerna i grupper
med högre socio-ekonomisk status, och ökar gradvis med minskande socioekonomisk status. För spädbarn som överlever t.ex. en väldigt för tidig födsel, kan
det också återstå konsekvenser i form av sämre chanser till god hälsa eller optimal
kognitiv utveckling.
Det övergripande syftet med denna avhandling är att bidra till kunskapen om
relationen mellan socio-ekonomisk status och födelseutfall, genom att fokusera på
moderns utbildning och klass samtidigt som en rad olika födelseutfall studeras.
Födelseutfall kan ses som en överföring av moderns hälsokapital till spädbarnet,
och denna överföring är påverkad av socio-ekonomiska faktorer. De mer specifika
syftena var att undersöka relationen mellan moderns utbildning och födelseutfall,
att studera en eventuell kombinerad påverkan av moderns barndomsklass och
vuxen klass på födelseutfall, och att utforska hur moderns arbetsvillkor kan bidra
till klasskillnader i födelseutfall. Studierna är populationsbaserade, och fokuserar
på enkelfödslar 1973-90.
Resultaten tyder på att utbildningsskillnaderna i födelseutfall ökade under denna tidsperiod, särskilt när man jämför barn till kvinnor med högst och lägst utbildning. Den så kallade låg-födelsevikts-paradoxen uppkom, som här innebär att barn
med låg födelsevikt har lägre risk för spädbarnsdödlighet när deras mödrar har
lägre snarare än högre socio-ekonomisk status. Paradoxen indikerar att fördelningen av orsaken till låg födelsevikt skiljer sig åt mellan de två grupperna.
Vidare visades att även om moderns vuxenklass hade större inflytande på födelseutfall än vad hennes barndomsklass hade, fanns en association mellan hennes
barndomsklass och låg födelsevikt samt neonatal dödlighet som var oberoende av
hennes vuxenklass. Att denna oberoende association uppkom för de två födelseutfallen som är närmast födseln, men inte för postneonatal dödlighet, tolkas som att
dessa utfall medieras via moderns kropp.
Slutligen tyder våra resultat på att moderns arbetsvillkor starkt bidrar till klasskillnader i ett flertal olika födelseutfall. Lägre grad av jobbkontroll, högre grad av
risker och högre fysiska krav var alla mer eller mindre associerade med risk för
spädbarnsdödlighet, låg födelsevikt, väldigt låg födelsevikt, sämre fostertillväxt,
för tidig födsel, väldigt samt extremt för tidig födsel. De tre dimensionerna av
arbetsvillkor hade olika samband med de olika födelseutfallen, vilket tyder på en
hög komplexitet i relationerna.
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List of publications
This thesis is based on the following papers:
Study I
Gisselmann, M.D. (2005) Education, infant mortality, and low birthweight in
Sweden 1973-1990: Emergence of the low birthweight paradox. Scandinavian Journal of Public Health, 33:65-71
Study II
Gisselmann, M.D. (2006) The influence of maternal childhood and adulthood social class on the health of the infant. Social Science and Medicine
63(4):1023-1033
Study III
Gisselmann, M.D. Infant mortality: To what extent can social class inequalities be explained by maternal working conditions?
(Forthcoming as a book chapter in le Grand, Halldén and Hellgren (Eds.):
Ethnicity, Equality and Justice. Beyond the Paradigms of Recognition and
Redistribution, Cambridge Scholars Publishing)
Study IV
Gisselmann, M.D & Hemström, Ö. The contribution of maternal working
conditions to socio-economic inequalities in birth outcome
(Submitted)
All papers are reproduced with permission from the copyright holders.
8
Introduction
The overall aim of this thesis is to contribute to the understanding of the
association between socio-economic status and the health of infants, and
this is accomplished by a focus on maternal socio-economic indicators.
The background for this interest is the persistent phenomenon that infants
born to women with higher socio-economic status generally have more
advantageous births outcomes, such as higher birthweight and lower mortality.
With the development of the Swedish welfare system, inequalities in
birth outcome were thought to have disappeared and this was in fact stated
in the Black Report (Black, 1980). But while the rates of perinatal and
infant mortality are internationally low in Sweden (Vallin, Meslé, & Valkonen, 2001), the association between socio-economic status and birth outcomes such as infant mortality do not seem less obvious than in other
countries as was suggested in the early 1990’s (Köhler, 1991).
Mechanisms of inequality are likely to be many and complex. In this
thesis I have tried to apply the thinking of causal pathways. I view socioeconomic indicators as being embedded in the structure of society, with
each indicator possibly reflecting a different set of causal pathways, resulting in inequalities in birth outcome. This means that while class might
work through its associations with working conditions to influence class
inequalities in birthweight, education may work through abilities or knowledge to influence educational inequalities in preterm birth.
This thesis will merely start to outline possible causal pathways. These
pathways are likely to change over time. The mechanisms resulting in inequalities in infant mortality in early 20th-century Sweden with a much
higher rate are not necessarily the same as later in the century when the
pattern of causes of death was different. In order to avoid the assumption
that mechanisms are the same across societies and that their relative importance is the same independent of the patterns of outcome, the focus of empirical and theoretical work has been Sweden in the last quarter of the 20th
century (1973-1990). This interest in pathways within Sweden entails that
country comparisons will not be considered.
Why does an infant born to a mother with lower socio-economic status
have a higher risk of for example preterm birth in our comparatively egalitarian and rich welfare society? Before this question is explored further I
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will emphasise its importance, both in terms of what makes birth outcome
different than many other health outcomes, and in terms of justice.
Theoretically, this thesis will try to extend earlier theorising by explicitly consider the link between maternal health and biology, and infant
health. As a consequence of this, the entire maternal life course is interesting in the study of inequalities in birth outcomes. Also, the focus on the
mother-infant link highlights the transfer of health between the generations. This point of transition is a biological link and, as will become clear
in this thesis, is highly influenced by socio-economic factors.
Birth outcome differs from most health outcomes in that although the
socio-economic indicators measured are those of the parent(s), the consequences of these indicators are extended to another individual. While this
clarification might seem trivial, it is of significance for the issue of justice.
Differences in health between social groups are often referred to as health
inequalities. The term inequality may be used primarily to indicate differences between groups, and not necessarily to emphasise the case of moral
judgment implied by the term inequity (Kawachi, Subramanian, &
Almeida-Filho, 2002). I would argue, however, that socio-economic inequalities in birth outcome are unjust, regardless of which definition of equity is applied.
A commonly used reference in the area of justice is Rawls (1971), who
suggests that there are some basic liberties that should be available to all:
freedom of speech and thought, for instance. Daniels (1985) extends his
theories to health care, arguing that this should be considered a special case
because good health is required to enjoy the other basic liberties. In the
research field of health inequalities, it is often argued that inequalities due
to individual choices, like health behaviours, should not be considered
unjust (Whitehead, 1991). What characterises an informed and free decision is however debatable, and does not seem to be independent of social
structure (Vallgårda, 2006). Applying this line of argument to infants, who
can not in any way be held responsible for their state of health, death before the age of one necessarily deprives them of even the basic liberties
suggested by Rawls. Infants surviving disadvantageous birth outcomes,
suffering impaired sight or hearing or learning difficulties, clearly do not
have the same opportunities as healthier infants do. For these reasons,
socio-economic inequalities in infant health must be considered unjust1.
For infants and their parents, it is of the highest priority that any avoidable adverse infant health outcomes be eliminated.
1
The title of the thesis, ”The first injustice”, was first used by Gortmaker and Wise 1997.
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Outline of thesis
The purpose of the introductory chapters is to provide a context for this research project, and to make the studies more accessible to readers who are
not familiar with the research area. The outline of the thesis is as follows:
Firstly, birth outcomes and socio-economic indicators are described and
discussed. Secondly, socio-economic inequalities in birth outcome in Sweden and the other Nordic countries are described, and previous explanatory
approaches are discussed. Thirdly, an explanatory framework of socioeconomic inequalities in birth outcome is outlined. Finally the studies constituting the thesis are presented, followed by a discussion of the thesis as a
whole. A short list of conclusions completes the introductory chapters.
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What are socio-economic inequalities in birth
outcome?
Social impact on infant health is widely recognised. Focus from a sociological perspective is often on the infant mortality rate, which has long been
used as a measure of living standard in populations. Social researcher Harriet
Martineau mentions a high proportion of infant deaths as “a most unfavourable symptom of society” (Martineau, 1838, p171), and it is still considered
to be a valid measure of population health (Reidpath & Allotey, 2003). Also
birthweight is used worldwide as a health indicator in infants (World Health
Organisation, 2005).
As previously recognised, mortality outcomes do “not adequately reflect
the disease burden in the living population” (Fritzell & Lundberg, 2006, p 9),
and there is good reason to expand research on social inequalities in infant
health to birth outcomes beyond both infant mortality and birthweight. I will
return to this issue after a short presentation of the birth outcomes examined
here.
Birth outcomes in this thesis
The birth outcomes studied in this thesis are defined in Table 1, and can all
be found in the Swedish Medical Birth Register.
Mortality outcomes are classified according to the age of the foetus or infant, from late foetal death to postneonatal mortality. The most commonly
used indicator in social research has been infant mortality, the death of infants born alive that dies before the first birthday. The major causes of infant
death in the developed world, where infant mortality rates are low, are congenital malformation and problems related to preterm delivery (Nordström,
Cnattingius, & Haglund, 1993). The death of a foetus after 28 completed
weeks of pregnancy is classified as stillbirth (National Board of Health and
Welfare, 2002) (the precise week differs between countries).
Gestational age refers to the length of time the foetus has grown inside the
uterus. The shorter the gestational age, the higher the mortality risk. Those
that survive a very preterm birth tend to have permanent deficiencies in
growth and problems with visual and hearing impairment as well as neurocognitive development (Kramer, 2003; Slattery & Morrison, 2002). Problems related to cognitive development are the most common, and can in the
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long run affect possibilities to achieve good school results and higher education (Colvin, McGuire, & Fowlie, 2004). There is a tradition in health research, in line with general welfare research in the Scandinavian countries,
of focussing on disadvantageous rather than advantageous outcomes (Fritzell
& Lundberg, 2006), and this has generally been the case for birth outcomes
as well. Gestational age is often dichotomised into term and preterm, and the
categories of very or extremely preterm are sometimes used as well.
Not all infants grow at the same rate during gestation, especially during
the latter part of pregnancy. Researchers are interested in defining those infants with intrauterine growth restriction (IUGR), often indicated by a
Table 1. Definition of birth outcomes
Foetal deatha
A fetus dies in the womb or at delivery, after pregnancy
week b 28c
Early neonatal death
An infant alive at birth dies before 7 completed days
(day 0-6)
Perinatal death
Foetal deaths and early neonatal deaths taken together
Neonatal death
An infant alive at birth dies before 28 completed days
(day 0-28)
Post-neonatal death
An infant alive at birth dies after 28 but before 365
completed days (day 29-365)
Infant death
An infant alive at birth but dies before 365 completed days
(day 0-365)
Low birthweight
<2500 g
Very low birthweight
<1500g
Preterm
Born before 37 completed weeks of gestation
Very preterm
Born before 32 completed weeks of gestation
Extremely preterm
Born before 28 completed weeks of gestation
Small-forgestational-age
(SGA)
At a given gestational week, those in the decile with the
lowest birthweight or two standard deviations below the
mean
a
Foetal death and stillbirth are considered interchangeable terms.
Aims at pregnancy weeks as traditionally starting at the beginning of last menstrual period
before conception (adding approximately 2 weeks before conception to foetal life)
c
This definition varies between countries
b
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definition of small-for-gestational-age (SGA). A definition of SGA means
that the infant is either two standard deviations below the mean or belongs to
the 10% with the smallest amount of foetal growth, in relation to others born
the same week (Källén, 1995; Savitz, Hertz-Picciotto, Poole, & Olshan,
2002). Foetal growth restriction is associated with increased risk of foetal
death, as well as mortality risk after birth. There is a difficulty in defining
which of the SGA infants that are truly growth-restricted, an area in which
diagnostic tools are making advances (Haram, Softeland, & Bukowski,
2006). Infants with the lowest foetal growth rate are not a specific group, but
represent one end of a continuum. For many SGA infants it may be that they
are constitutionally small, which should not be considered “truly” growthrestricted. Moreover, while most SGA infants are born at term, risk factors
for SGA may not be the same for preterm and term SGA infants (Clausson,
Cnattingius, & Axelsson, 1998).
Birthweight is a well researched birth outcome. Generally, lower birthweight is associated with higher mortality risk in the infant period. It has
also been linked to an increased risk for various health problems in adult life,
such as coronary heart disease (Godfrey & Barker, 2001), as well as more
disadvantageous cognitive outcomes (Spencer, 2003). The concept of low
birthweight is very common, referring to a birthweight less than 2500g, with
lower weight categories sometimes being used as well.
In addition to the birth outcomes in this thesis, there are other pregnancy
or reproductive outcomes. For instance the term pregnancy outcome which
includes issues of fecundity (the capacity to give birth), fertility (actual reproductive outcome) and early pregnancy loss (Savitz, Hertz-Picciotto,
Poole et al., 2002).
Causes of disadvantageous birth outcomes differ
Gestational age, foetal growth and birthweight are all interrelated and associated with mortality risk. It is believed that foetal growth rate might contribute to setting the time of delivery, thereby affecting gestational age. Also,
gestational age and foetal growth rate can be seen as two major pathways to
achieved birthweight, which they precede in time. Partly because birthweight
can be seen as a result of gestational age and foetal growth, it has been criticised for being an imprecise endpoint (Wilcox, 2001). And indeed, when
there is any interest whatsoever in aetiological processes and data are available, there seems to be no reason to settle with only studying birthweight.
However, availability and reliability of birthweight have been high compared with the other two outcomes, both over time and worldwide. Birthweight may be seen as a composite measure of infant health, even if it is not
necessarily on the causal pathway to adverse birth outcomes such as infant
mortality or ill health later in life (Spencer, 2003).
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Although the above birth outcomes are interrelated, there are good reasons to study them as separate entities. In the context of this thesis the prime
reason is that their causes seem to be different, even though knowledge
about aetiological processes behind birth outcomes is limited (Kramer,
2003), indicating that the pathways between socio-economic factors and
birth outcomes may differ.
Research on birth outcomes needs to separate single births (singletons)
from multiple births such as twins or triplets. Birth outcomes for multiple
births are different in several ways from those of singletons; for example,
they generally have lower birthweight, shorter gestational age and higher
mortality which can be said to be caused by the multiplicity. For this reason
only singletons are studied in this thesis.
Beyond the scope of this thesis, causes of adverse birth outcomes is of
further interest because they may have different consequences later in life.
Birth outcomes as health indicators
Not all birth outcomes are strictly speaking health outcomes as such, but
they can all be considered as health-relevant in that they are very strongly
related to health and are therefore often used as health indicators. In addition
to this, they are not only health-related but also seem to carry health potential. As described above, disadvantageous birth outcomes are related to adult
health outcomes; we can thus talk about health potential.
With the concept of health status, we are usually referring to something
we can measure in the present. We may define an infant as growth-restricted
at term and use this as a health indicator. While it seems that something has
happened to produce this result, we are not sure how or why, or even what
has happened. But it does seem to indicate that something has happened
inside the body, even if we are not be able to measure precisely what it is. In
one line of research this is called “biological programming” (Barker, 1991),
which can be perceived as quite deterministic, and in a wider context this
relatively new research field has become known as “developmental origins
of disease” (For further discussion on this, see “Mapping out an explanatory
framework”). While the concept of health potential perhaps gives the impression that outcomes later in time is of interest, life course research has
used the concept health capital (Kuh, Power, Blane, & Bartley, 2004), something that might seem to indicate that there is a longer life course to refer
back to in time.
In this thesis, the concept of health potential will be used as something
that has already happened in the body, and may influence health or susceptibility to health problems further from this insult in time as well, thus compatible with the concept of health capital. That foetal and infant life “provides a health potential which continues to act on later health during the
entire life course has substantial credibility” (Elstad, 2000, p 95).
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Socio-economic indicators
The most commonly used socio-economic indicators are income, education
and social class. In empirical research it has occasionally been assumed that
all these indicators measure the same underlying concept, for example status
(see for example Marmot, 2004), and are therefore interchangeable. This
thesis will use the term socio-economic status as a collective term for the
socio-economic indicators, rather than referring to any specific concept of
status or prestige. There have also been strong arguments raised against the
presumed interchangeability of socio-economic indicators (Geyer, Hemström, Peter, & Vågerö, 2006); here they will be assumed to reflect different
dimensions of socio-economic inequalities, thus causal pathways between an
indicator and a specific birth outcome may vary.
Level of education is always tied to the individual. Education may increase cognitive abilities (Erikson, 2001), for example the ability to process
information, in terms of amount and complexity. It may also increase the
ability to solve problems. These abilities may improve decision-making or
increase access to other resources. For example, a higher education will be
necessary for many highly qualified jobs, which will generally be better paid
and have healthier working conditions.
Income has been suggested as the best measure of material conditions
(Erikson, 2001). It is often measured at a household level, which tends to
elevate the income of married and cohabitants compared with singles, and
household income is often adjusted for number of adults and children living
in the household. As a resource, income can be considered a very general
resource, because it is possible to transform it into other resources. Because
of this characteristic, those with a higher income can be thought of as having
more control over life (Erikson, 2001). It is also the most dynamic indicator,
in that it varies over time more than the others do. In determining living
standard it plays a large role in terms of housing, consumption and recreation
(Income is not used empirically in the studies constituting this thesis).
The most complex socio-economic indicator is social class. The socioeconomic classification system in Sweden is intended to capture life
chances, and it has been argued that it is close to Weber’s class theory
(Fritzell & Lundberg, 1994). The starting point of the Swedish classification
system is the occupational relation, with the first distinction made between
employees and self-employed. From the latter the category of farmers can be
extracted, providing two groups of self-employed: farmers and all other selfemployed. Employees are divided into manual and non-manual workers.
These groups can also be more finely categorised: manual workers can be
split into more and less qualified, and into the service or manufacturing work
sector. Non-manual workers are hierarchically classified into higher, middle
and lower. This system has been applied in censuses in Sweden since 1980,
and in this year the classification resulted in 18 groups, including 6 groups
16
not gainfully employed or unclassifiable for other reasons (Andersson, Erikson, & Wärneryd, 1981; Statistics Sweden, 1982).
The indicators are interrelated so that, for example, higher education is a
common prerequisite for achieving occupations classified as higher social
class. Therefore, the association between education and health, for example,
can be seen as partly mediated by social class (Lahelma, Martikainen, Laaksonen, & Aittomaki, 2004). When using all three indicators in the same
model, Geyer et al. find a remaining association with mortality for each indicator when the others are controlled for (Geyer, Hemström, Peter et al.,
2006), which has previously been found for income (net of education and
social class) and self-rated health (Fritzell, Nermo, & Lundberg, 2004).
The Swedish socio-economic classification in itself also contains criteria
for level of qualifications normally required for a position, whereby level of
education is quite strongly tied to social class – at least if the measurement
of class is based on the individual occupation. As mentioned above, education is always measured on an individual level and income either on an individual or household level. How women’s social class should be measured is,
however, a more controversial issue.
Measuring social class for women
In health-related research the individual-based concept of occupational class
is sometimes used, for example when studying working conditions. In contrast, the concept of social class is used to denote more than simply aspects
of work. This concept goes back to theorists such as Marx and Weber, and
has been well debated since their time. It has been argued that social class is
a characteristic of the household rather than the individual (Erikson, 1984a;
Erikson, 2006). Concepts of class are usually based on occupation, which is
assumed to reflect the position both on the market and at work (Erikson,
1984a). Historically, not all groups of women have had the same strong connection with the labour market as most men have. Whereas a large proportion of working class women have been in paid labour, the proportion of
middle-class and especially upper-class women has been much smaller
(Ohlander, 1994). In agriculture, women in farming families have generally
not been considered to be gainfully employed (Erikson, 1984b). For married
couples, social class has traditionally been considered best assessed using the
occupation of the husband. Going back in time, this may have been an adequate reflection of life chances for the family, especially for the middle and
upper classes (Farmers have probably worked together, and among manual
workers women have often been single providers (Ohlander, 1994).
Since the latter part of the 20th century, it has been argued that social
class is best assessed using the dominance method. According to this, the
person with the dominant social class position should be used for classification of the entire household (Erikson, 1984a). Once both occupations have
17
been considered, this method ignores rather than takes into account the occupational class of the individual with the subordinate occupation (Heath &
Britten, 1984). Because men generally have dominant labour market positions in Sweden, households with two adults are usually assigned the social
class of the husband. Thereby, non-cohabiting women are also disproportionally assigned to the lower classes (Arber, 1997). While some women in
the high non-manual class are highly educated and have high-status jobs
such as medical doctors, others in the same category have low education and
a manual job, or are unemployed. Since many relationships do not last a
lifetime, and a woman’s education and work experience is more important
for her labour market position than that of her husband, these factors must
also be considered very important for a woman’s life chances after a separation.
In studying the relation between class and health, different ways of measuring social class give different results. An individual measure of class,
compared with a household measure, has been shown to give a larger class
difference in limiting long-standing illness for women (Arber, 1997), probably because this health outcome is highly associated with working conditions. A general measure like self-rated health, on the other hand, was shown
to give larger differences with a household measure for women (Arber,
1997). This may reflect the importance of material living conditions (for
self-rated health), which has been shown to be more related to the occupational class of a male cohabitant (Erikson, 1984a).
In the field of health inequalities, it has been suggested that a theoretical
choice of indicator should be made based upon the explanatory pathway of
the inequalities (Bartley, Sacker, Firth, Fitzpatrick, & Lynch, 2000). From
the perspective of sociology, this may not be considered an optimal method
(see for example Vågerö, 2000), since health inequalities are not necessarily
the most important aspect of social class. In spite of this, the discriminatory
power of indicators is sometimes used to strengthen the argument of a
household rather than an individual measure (Erikson, 2006; Erikson &
Goldthorpe, 1992).
Applying these arguments specifically to the study of birth outcomes,
there are reasons to emphasise the maternal aspects of social class. Firstly, it
can be argued that foetal and infant health are associated with maternal
health. The maternal body accommodates the foetus, and the infant continues to be nourished straight from the maternal body after birth for as long as
breastfeeding is practiced. Secondly, the mother is persistently the predominant caretaker during the largest part of infancy in Sweden. The paternal
proportion of parental leave during the first year is increasing, but this thesis
focuses on the time period 1973-1990 when only 5% of the days of parental
leave during infancy were used by the father (Statistics Sweden, 2007). Using women’s own occupations as the basis for class will result in classes
composed of women with more similar everyday living conditions. In this
18
thesis, women’s own occupational class is referred to as “social class” in
Studies II and III, and as “class” in Study IV. The two last studies are focused on working conditions, which is a case when also advocates of the
household method recommends using women’s own occupation (Erikson,
1984a); I recognise that this might underestimate the role of the household in
terms of material living standard, according to the argument by Dale et al
(Dale, Gilbert, & Arber, 1985).
19
Previous Nordic research
Research relying on historical statistics estimates that infant mortality in
Sweden around the year 1750 was over 200 per 1000 births. This had been
reduced by 50% already in the late 1880s, and had decreased to around 60
per 1000 births around 1920 (Rydell, 1976). According to this estimate, most
of the decrease (about 75%) in infant mortality occurred before medicine
intervened with its modern therapeutic arsenal. Improvements have been
attributed mainly to better living standards; nutrition, housing, water, hygiene and education are usually mentioned. All this is believed to have increased infants’ survival ability and reduce contamination by disease agents
(Köhler, 1991; Rydell, 1976). With the reduction of infant mortality, the
domination of postneonatal deaths until the 1930’s is followed by a domination of neonatal deaths (Figure 1).
In international comparisons, birth outcomes in Sweden have been
amongst the most favourable in the world since the 1950s. The internationally low level of infant mortality (Table 2) has especially been lauded in
Swedish Public Health Reports (National Board of Health and Welfare,
1994; 1997; 2001).
Figure 1. Infant and perinatal mortality in Sweden 1915-1977. Source Karlberg &
Erikson (1979), reproduction by National Library of Sweden.
20
That infant health also varies within the Swedish population according to
socio-economic status was already documented eighty years ago when Rietz,
a German researcher, noted that the infant mortality rate decreased with increasing paternal income (Rietz, 1930).
Modern research shows socio-economic differences in infant mortality in
an area of Stockholm during the years 1878-1925 (Macassa, 2004). Recent
studies also show how social class differences in diarrhoea mortality rates
for children under 2 increased when the higher classes improved their living
conditions, and how the gap decreased when the lower classes were also
reached by these improvements (Burström, Macassa, Öberg, Bernhardt, &
Smedman, 2005; Burström & Öberg, 2006).
Table 2. Infant Mortality in some European Countries
1950
1965
1980
1995
Romania
123.7
46.4
29.8
22.5
Poland
112.2
43.2
25.5
13.6
Bulgaria
106.1
32.0
19.6
16.2
Portugal
99.2
66.2
24.0
7.4
Italy
68.1
35.6
14.5
6.2
Belgium
53.6
24.6
11.9
7.2
Finland
42.3
16.7
7.3
4
Greece
40.3
34.7
17.6
7.7
United Kingdom
32.1
19.9
12.1
6.1
Denmark
31.4
18.1
8.4
5.9
Norway
27.2
15.9
8.1
4.4
Netherlands
26.7
14.6
8.5
5.6
Sweden
21.9
13.4
7.1
4.2
Source: Vallin, Mésle, & Valkonen (2001)
21
The Nordic countries are all relatively egalitarian, and have low levels of
disadvantageous birth outcomes. Patterns of socio-economic inequalities in
birth outcome seem similar in the Nordic countries, and research has generally been carried out on the entire population due to the medical birth registries in these countries from around 1970.
In the next section is a short presentation of descriptive results from studies of socio-economic inequalities in birth outcome in the Nordic countries
for the years 1992-2006. In a series of studies, a Swedish research group
explored socio-economic differences in birth outcome using an indicator of
privileged versus underprivileged (Ericson, 1984; Ericson, Eriksson, Källén,
& Zetterström, 1990; 1993; Ericson, Eriksson, & Zetterström, 1984). This
classification was a composite measure, based on a mixture of socioeconomic and social indicators such as income, cohabiting status and maternal occupation. Because interpretations and comparisons are difficult, these
studies will not be included in the presentation.
In some of the Swedish studies below, social class is based on household
or cohabitant occupation rather than maternal occupation; when this is the
case it will be noted in the text.
Mortality
The most frequently studied birth outcome in the social sciences is infant
mortality. At least ten studies show higher risks for infant mortality for infants born to women with lower education (Arntzen, Magnus, & Bakketeig,
1993; Arntzen, Moum, Magnus, & Bakketeig, 1996a; 1996b; Arntzen,
Samuelsen, Daltveit, & Stoltenberg, 2006; Bakketeig, Cnattingius, & Knudsen, 1993; Cnattingius & Haglund, 1992; Gisselmann, 2005; Haglund, Cnattingius, & Nordström, 1993; Helweg-Larsen, Olsen, & Madsen, 1999;
Hemminki, Merilainen, Malin, Rahkonen, & Teperi, 1992; Olsen & Madsen,
1999; Valkonen, Martelin, Rimpelä, Notkola, & Savela, 1993). Most studies
separate neonatal and postneonatal mortality, with the median of odds ratios
at 1.4 and 1.8, respectively, although not all odds ratios are statistically significant. In some studies only the lowest educational category has a higher
risk than the other groups, and in some there is a gradient. There are also
some studies in which there is a gradient, but it is not carried on to the highest educational group (Haglund, Cnattingius, & Nordström, 1993; Hemminki, Merilainen, Malin et al., 1992; Valkonen, Martelin, Rimpelä et al.,
1993), with odds ratios around 1.4 for neonatal and 1.2-1.4 for postneonatal
mortality.
There are consistently higher risks for infant mortality for infants born to
manual workers than to non-manual workers, and this is often statistically
significant (Arntzen, Moum, Magnus et al., 1996b; Gisselmann, 2006;
Gissler, Rahkonen, Järvelin, & Hemminki, 1998; Helweg-Larsen, Olsen, &
Madsen, 1999; Valkonen, Martelin, Rimpelä et al., 1993), as well as when
22
household (Haglund, Cnattingius, & Nordström, 1993) or mother’s cohabitant (Leon, Vågerö, & Olausson, 1992) was used to assign social class. Considering the different measures of class and the different reference groups, it
is not clear how the odds ratios should be compared, but they vary between
1.2 and 1.6 for these studies and neonatal, postneonatal and total infant mortality.
One study includes income (probably maternal) as a socio-economic indicator. Contradictory to most findings, neonatal mortality seems to increase
with increasing income, while there is no trend for postneonatal mortality
(Arntzen, Moum, Magnus et al., 1996b).
Foetal death seems consistently related to socio-economic status. Lower
maternal education was associated with higher risk for stillbirth in all the
reviewed studies (Arntzen, Magnus, & Bakketeig, 1993; Cnattingius &
Haglund, 1992; Haglund, Cnattingius, & Nordström, 1993; Hemminki,
Merilainen, Malin et al., 1992; Olsen & Madsen, 1999; Valkonen, Martelin,
Rimpelä et al., 1993) and median of odds ratios was 1.6. A manual social
class was related to higher risk for foetal death than a non-manual class, both
when this was based on maternal occupation (Helweg-Larsen, Olsen, &
Madsen, 1999; Stephansson, Dickman, Johansson, & Cnattingius, 2001;
Valkonen, Martelin, Rimpelä et al., 1993) and on household occupation
(Cnattingius & Haglund, 1990; Haglund, Cnattingius, & Nordström, 1993)
with odds ratios between 1.4 and 2.0.
Perinatal death seems to be a less common choice of birth outcome, but
both lower maternal education (Hemminki, Merilainen, Malin et al., 1992;
Valkonen, Martelin, Rimpelä et al., 1993) and manual, compared to nonmanual, social class (Gissler, Rahkonen, Järvelin et al., 1998; Valkonen,
Martelin, Rimpelä et al., 1993) has been linked to an increased risk for perinatal death (odds ratio 1.3).
Birthweight
Birthweight has persistently been shown to be associated with maternal education, so that lower education is related to a higher proportion of infants
with low birthweight with odds ratios of 1.7-2.7 (Arntzen, Samuelsen, Magnus, & Bakketeig, 1994; Gisselmann, 2005; Helweg-Larsen, Olsen, &
Madsen, 1999; Hemminki, Merilainen, Malin et al., 1992) or lower birthweight (Bakketeig, Cnattingius, & Knudsen, 1993; Koupilova, Vågerö,
Leon, Pikhart, Prikazsky, Holcik et al., 1998).
Compared to higher or middle non-manuals, manual workers tend to have
an increased risk of giving birth to infants with low birthweight
(Gisselmann, 2006; Helweg-Larsen, Olsen, & Madsen, 1999) also when
household class was used (Vågerö, Koupilova, Leon, & Lithell, 1999).
One study suggests a tendency for birthweight to increase with maternal,
but not paternal, income (Arntzen, Samuelsen, Magnus et al., 1994).
23
Gestational age and foetal growth
A higher risk for preterm birth has been shown in infants born to women
with lower education with an odds ratio of 1.6 (Hemminki, Merilainen, Malin et al., 1992; Koupilova, Vågerö, Leon et al., 1998), and to manual in
comparison to non-manual workers (odds ratio 1.25) (indicated by household) (Vågerö, Koupilova, Leon et al., 1999). One study indicates that low
maternal education increased the risk for SGA for preterm, but not full term,
infants (Clausson, Cnattingius, & Axelsson, 1998).
Explanatory approaches
From the overview of the Nordic studies above, it is clear that the main interest has been socio-economic inequalities in infant mortality. A reason for
this focus might be the long tradition of using this as a social indicator.
In the articles reviewed above, theoretical explanations are generally limited to a sentence or two in which possible reasons are stated. Living conditions, lifestyle and smoking, as well as occupation or work are the most
commonly mentioned theoretical explanations, followed by stress, access to
health care and material deprivation. Of these explanations, only the health
behaviour smoking is sometimes described and discussed in more detail, and
empirical explanation seems limited to this (see for example Haglund, Cnattingius, & Nordström, 1993). That is, other factors are adjusted for but are
not considered mediating factors.
Some conclusions can actually be drawn from these studies, even if they
were not made by the authors. For example, from a study interested in the
influence of marital status on birth outcome, it is clear that educational differences are not erased when social class or income is adjusted for. This in
fact increases the odds ratios, while class differences on the other hand decrease when education and income are adjusted for (Arntzen, Moum, Magnus et al., 1996b).
Explanations of inequalities in birth outcome within developed countries
that have been discussed in other countries have been related to birthweight
and selection effects. In the first case, the research question was how much
of infant mortality differences were mediated by birthweight (Leon, 1991).
Considering that birthweight is the result of both foetal growth and gestational duration, and that the causal factors behind these are not identical,
adjusting for birthweight perhaps does not contribute much to the understanding of the causal pathways between socio-economic factors and infant
mortality. A discussion of selection effects in relation to birth outcome was
based on an article showing that women with better “personal qualities” in
term of physique, health and height were upwardly mobile in their marriages, and that they also gave birth to infants with better life chances
24
(Illsley, 1955). However, the main interest in the selection debate was perhaps the gradient in adult health rather than the explanation of inequalities in
infant health per se (see for instance Blane, 1985; Blane, Harding, & Rosato,
1999; Fox, Goldblatt, & Jones, 1985; Rahkonen, Arber, & Lahelma, 1997),
and the significance of health selection could vary by outcome.
From a medical point of view, there has – naturally and rightly so – been
a focus on the prevention of disadvantageous birth outcomes for the individual, mainly through identification of “risk factors”. However, some of these
can not be considered as such from an aetiological or sociological perspective, and would better be called risk indicators than risk factors. An example
of a problem with relevance for the aetiological perspective is prior preterm
birth, which is regarded as a risk factor for another preterm birth. However,
this does not reveal anything about why there was a prior preterm birth, or
why another might occur. An example of when a sociological perspective is
relevant is the issue of first parity, the firstborn infant to a woman. Infants of
parity one are usually smaller than laterborns, and have an elevated mortality
risk. However, having a first birth is not a medical problem, and there are
even indications that firstborns have other advantages in terms of educational achievement (Modin, 2002a; Walldén, 1992) and lower mortality in
adulthood (Modin, 2002b). In addition to this, parity is not independent of
socio-economic status (see demographic factors, below). An example of
when this is problematic is the determination of SGAs. If parity is not considered, a higher proportion of firstborns than laterborns are categorised as
SGA. This confusion about risk indicators and risk factors has had an unfortunate tendency to be carried over to both overviews and empirical research
(see for example Kramer, 1987; 2003). Also, the medical perspective usually
focuses on the most disadvantaged rather than on the whole scale of social
stratification (see for example Kramer, Seguin, Lydon, & Goulet, 2000).
Demographic factors
There are at least two demographic factors of prime interest for socioeconomic differences in birth outcome: maternal age and parity. Mortality
usually has a u-shaped association with maternal age, so that infants born to
very young mothers as well as those born to mothers of higher age have increased risks. For example, recent results with data of high quality showed
that the risk of foetal loss is significantly higher when maternal age exceeds
35 (Andersen, Wohlfahrt, Christens, Olsen, & Melbye, 2000). A maternal
age exceeding 35 is also linked to higher risk for SGA (Clausson, Cnattingius, & Axelsson, 1998), as well as very and mildly preterm birth (Ancel,
Saurel-Cubizolles, Carlo Di Renzo, Papiernik, & Breart, 1999). Parity is also
associated with birth outcome. Infants of first parity tend to have higher mortality than those of second parity, and there is a slight increase with higher
parities. Birthweight tends to be lower for the firstborn than for higher pari25
ties. While the increased risk for disadvantageous birth outcome with increased maternal age is assumed to be caused by the aging process itself,
there are indications that the increased risk for younger women might be
more social in origin (Markovitz, Cook, Flick, & Leet, 2005). Moreover,
associations between socio-economic indicators and birth outcome have
been shown to vary across parities, the relation between maternal education
and post-neonatal mortality for instance (Arntzen, Moum, Magnus et al.,
1996a), indicating complex associations.
This is of interest here because maternal age and parity are not evenly distributed over socio-economic indicators. Women with a working-class background or lower adult socio-economic status tend to give birth at younger
ages than do women with a non-manual background or higher adult socioeconomic status. Maternal age and parity are often adjusted for because researchers are looking for differences beyond these factors. From a sociological perspective, they can also be viewed as part of the socio-economic differences. Although these factors can make a contribution, it does not seem to
be very large and how it affects socio-economic differences is not always
apparent. The younger mothers, more common in manual classes, will give
birth to infants with a higher mortality risk, but so will the older mothers
who are more common in the non-manual classes. The extent to which these
factors act as mediator of inequality, or can be considered confounding, depends on the composition of the population under study and on the research
question at hand. Therefore, demographic factors are considered here, although another option would have been to include them in the explanatory
framework outlined in the next chapter.
26
Mapping out an explanatory framework
The purpose of this chapter is to map out a suggested explanatory framework
for socio-economic inequalities in birth outcome. This may begin to answer
the question of how socio-economic status exerts an influence on birth outcome. To a lesser extent, the relative importance of pathways will be commented on.
As is the case with socio-economic differences in health in general
(Lahelma, 2006), those in birth outcome have not been explained all too well
(Slattery & Morrison, 2002). From a sociological perspective, there has been
an interest in welfare institutions (Wennemo, 1993), access to care
(Gortmaker & Wise, 1997) and racial differences (see for example North &
MacDonald, 1977). The birth outcome in focus has been infant mortality,
and interest has rarely been extended to gestational age or foetal growth, for
example. As was concluded above, the mediator receiving the most attention
in inequalities in birth outcome within the Nordic countries seems to have
been smoking. During the time period when most of the Nordic articles in
the review were published, there was a great interest in the empirical explanation of health inequalities in adults. Factors found to be important for
adults included working conditions in adult life and living conditions in
childhood such as economic difficulties (Lundberg, 1991a; 1993). Thus,
fields and disciplines of research have not been brought together, and the
explanations commonly applied to adult health inequalities do not seem to
have been applied to birth outcome to the same extent.
One reason for the scarcity of sociological explanations for health inequalities among infants might be that the link between maternal and infant
health has not been explicitly dealt with. If this is not considered, there is no
rationale for applying explanations for adult health inequalities.
The link between maternal and infant health
Already 60 years ago, it was suggested that maternal and child health should
be studied together, especially for birth outcomes early in the life of the infant (Morris & Heady, 1955), and this has again been pointed out in more
modern times (Wise, 2003). Nonetheless, there is a complexity in the association between maternal and infant health. On the one hand, maternal and
infant health are not the same outcome. A known factor detrimental to infant
27
health, which is a likely mediator of socio-economic inequalities in infant
health, is smoking (Kramer, Seguin, Lydon et al., 2000; Nordström, 1995).
Smoking certainly influences maternal health, but not to the same extent as it
influences infant health, and not via the same mechanisms. On the other
hand, the maternal body mediates many birth outcomes, measured at time of
birth: birthweight, gestational age, and foetal growth, for example. Although
neonatal and postneonatal deaths entail more time outside the womb, a large
part of the causes of these deaths precede birth in time, such as congenital
malformation and immaturity-related causes. With the practice of breastfeeding, the maternal body may continue to exert an influence on infant health
after birth. Thereby, many outcomes are mediated via the maternal body, and
are very likely to be influenced by maternal health. This has been expressed
as “during pregnancy and early childhood, children literally “embody” (biologically incorporate) the social status of their mother” (Koenen, Lincoln, &
Appleton, 2006, p 3000). This will be addressed in Study II.
Thus, there is complex duality of infant and maternal health, and this link
needs to be addressed in the context of socio-economic inequalities in birth
outcome. For example, when considering how socio-economic status might
influence infant health, we may at least theoretically try to separate influence
mediated through the maternal body from that which is not.
The framework
When the link between maternal and infant health is acknowledged, the entire maternal life course is of interest for birth outcome. The socio-economic
circumstances over the life course might be thought of as a social career. Just
as individuals have a socio-economic career, they can have a health career
(Vågerö, 1998; Vågerö & Illsley, 1995). If your social career includes school
results, educational choices and attainment, as well as occupational career,
every step of the way can be affected by your physical and mental health.
Likewise, success in your social career can have a positive impact on your
health, while drawbacks tend to increase the risk for health problems, as well
as stress-related unhealthy behaviours such as smoking and alcohol intake.
Thus, social and health development are intertwined.
The explanatory framework presented here is intended to be an outline
rather than a complete description of all possible pathways. Along the pathways sketched here, infant health may be affected by maternal socioeconomic factors, directly and via maternal health. Socio-economic indicators are not methodically presented separately, mainly because this would
increase the ambition and size of this chapter. Pathways of special relevance
to the discussion of the studies in this thesis are somewhat more elaborated
than others.
28
In order to assist the reader, a simplified model is provided. The orientation follows the co-evolution of health development and social development
over the life course, as proposed by Vågerö and Illsley (1995). Some things
fall outside the model, such as ecological factors that rather provide a context for the model, while others seem involved with all the boxes in the
model (see for example the section on stress below). In the model, “health”
refers to health potential or capital as described above. The endpoints chosen
for the boxes are imprecise, but hopefully the model will serve its illustrative
purpose. In the text below, pathways (represented by arrows) in the model
will be referred to with numbers in brackets.
1
Childhood
Adult
social status
social status
9
4
2
Childhood
health
6
7
5
Adult health
3
Infant health
8
Figure 2. Pathways of maternal co-development of social and health careers and the
influence on infant health.
29
Ecological factors providing a context
It may be that some factors do not vary enough within Sweden, but still have
an influence on the level of disadvantageous birth outcomes. A comparison
between 18 OECD countries indicates the importance of public policy for
infant mortality rate (Wennemo, 1993). Studies in, for example, the US have
shown associations between income inequality and neonatal mortality
(Mayer & Sarin, 2005), infant mortality and low birthweight (Shi, Starfield,
Kennedy, & Kawachi, 1999). It has also been found that in states where
women’s political participation was higher, economic and social autonomy
were associated with birth outcomes such as low birthweight and infant mortality (Koenen, Lincoln, & Appleton, 2006). While the importance of context
must be researched at a higher aggregate level, the interest of this thesis lies
within a population. What determines the health of a population as a whole
is not necessarily comprised of the same factors that determine the distribution within the population (Rose, 1985).
During the past decade, the research area of neighbourhood context has
received a great deal of interest. That the community might contribute to
individual health over and above individual socio-economic risk factors has
been shown in several studies for adults (Kölegård Stjärne, 2005; Robert,
1999) as well as for birth outcome (Farley, Mason, Rice, Habel, Scribner, &
Cohen, 2006; Messer, Kaufman, Dole, Savitz, & Laraia, 2006). Unemployment level among adult men have been shown to influence the rate of very
low birthweight in Sweden and Norway (Catalano, Hansen, & Hartig, 1999).
Pathways of contextual influences that have been presented include an
underinvestment in infrastructure, not only in that which is directly healthrelated such as access to high quality care (Shi, Starfield, Kennedy et al.,
1999), but also in social, physical and human infrastructure (Daly, Duncan,
& Lynch, 1998). Another suggested pathway is stress, for example due to
economic situation (Catalano, Hansen, & Hartig, 1999), also at a community
level (Culhane & Elo, 2005).
Moreover, it has been argued that there is a stress originating from inequality per se, that it is a stressor in itself to occupy a subordinate position in
a hierarchical social system. The proposed mechanism entails that individuals with lower social status feel that they are less respected (Wilkinson &
Pickett, 2006) due to this. Every contextual system does not affect all members of a society in the same way; rather, it depends on what situations are
implied by the positions. In other words, the stress or health consequences
are not given (Sapolsky, 1998). The pathway of stress is presented separately
in a section below, since it is interesting as a mechanism in this context.
30
Maternal socialisation, social selection (1)
According to socialisation theory, the early and continued socialisation of
individuals forms their behaviours, attitudes and tastes (Bourdieu, 1990).
According to these preferences they choose diverse ways into the labour
market, developing various strategies according to their social class, and end
up being exposed to dissimilar working conditions and health behaviours
(Singh-Manoux & Marmot, 2005). This theory includes elements of
Bourdieu's theories of taste. He suggests that individuals are provided with a
set of dispositions, guiding our tastes and choices in accordance with our
previous experiences. In this way we have internalised the social world, and
thus our past stays with us in the form of “habitus” (Bourdieu, 1990). The
theory of socialisation can seem extremely inclusive. However, it could be
argued that it plays down the importance of material conditions, and it has
been criticised for not recognising the importance of social structure
(Kendall & Jianghong, 2005).
In this framework, socialisation concerns the mother, and may be viewed
as an underlying process. The theory is especially useful in its acknowledgement of the reproduction of socio-economic inequalities. An example
will be given on education, the reproduction of which is very important in
this process.
Children growing up in non-manual households, or whose parents are
more educated, go on to higher education and arrive at non-manual positions
to a much higher degree than other children do. Swedish research has identified two important steps in this process. Firstly, children from families with
a higher socio-economic status achieve higher grades in school. Secondly,
independent of these grades, a larger proportion of these children choose to
go on to higher education (Erikson & Jonsson, 1993). Their actual achievement may be a result of higher priorities given to schoolwork, as well as
better parental capabilities to support their children’s schoolwork in higher
socio-economic families. Reasoning behind the choice of educational path is
thought to mainly be differences in aspiration (Jonsson, 2004). Qualitative
research has indicated that interpretation of grades varies with parental
socio-economic status: while higher socio-economic status adults interpret
lower grades as their children having not performed according to their full
potential, lower socio-economic status adults make the interpretation that
their children are not able to do better (Jönsson, 1993).
The influence of socialisation is thus multifaceted. In the case of education, it can be viewed as a process of social selection whereby those with
socio-economically advantageous backgrounds achieve higher socioeconomic status in their adult life to a greater extent. The influence of childhood socio-economic status in inequalities in birth outcome is explored in
Study II.
31
Health in early maternal life: socio-economic inequalities (2)
Inequalities in mortality by parental socio-economic status are present already in infancy, and are also found in childhood (Vågerö & Östberg, 1989;
Östberg & Vågerö, 1991). In the 1980’s, class differences in mortality in
Sweden were found to be larger for 1-19-year-olds than for 25-65-year-olds
(Östberg, 1996). Social class differences in health status in childhood seem
to vary by outcome. A study examining three psychosomatic complaints in
Swedish schoolchildren found that headaches, but not stomach aches or difficulties falling asleep, were significantly related to the social class of the
household (Östberg, Alfvén, & Hjern, 2006). A study of traffic injuries
showed that children with a manual background have between 20% and 30%
higher risks in comparison with children in middle and higher non-manual
families (Hasselberg, Laflamme, & Ringbäck Weitoft, 2001). According to
the theory of biological imprinting, susceptibility is present in the body and
even if it is not measurable with today’s indicators, health potential may still
be important in childhood.
Health in early maternal life: biological imprinting (3)
Several theories are concerned with how childhood health might be of importance for adult health. One is that health in early life is of specific importance for adult health due to its timing in the developmental process. In utero
and during infancy there are several critical and sensitive periods of development. If development has been negatively affected at this time point, this
can not be compensated for later (Power & Hertzman, 1997). A focus of
interest here has been the influence of early life on adult circulatory health
outcomes (Barker, 1991). Some researchers interested in the foetal programming hypothesis claim that there is a functional adaptation (Barker,
1991; Worthman & Kuzara, 2005). The argument is that if you experience a
specific environment in utero, for example nutritional restriction, this will
give information about the environment in which you will be born into and
the systems in your body will adapt to this. The outcome will be favourable
if this is indeed what the environment is like, but if there is a mismatch it
will be detrimental to your health. However it might not be justifiable to
interpret all adjustments as something that necessarily have a predictive
value; they should perhaps be seen more as compromises in development
necessary for survival (Jones, 2005; Leon, 2004). In the well known example
of low birthweight and adult health problems, it has yet to be shown that the
original insult would give a better outcome in the case of starvation for it to
be of adaptive value.
Maternal foetal development is of interest as there are indications that the
ability to maximise a pregnancy outcome is formed during this period
(Lumey & Stein, 1997; Misra, Astone, & Lynch, 2005). It has been shown,
32
for example, that infants born to women who had a birthweight of less than
2000g have a higher risk of perinatal mortality (Skjaerven, Wilcox, Oyen, &
Magnus, 1997). (This influence can also be considered intergenerational,
which will be discussed in another section below.)
Theoretically, this influence of foetal development could in fact be limited to maternal reproduction capability. However, early maternal childhood
growth has been linked to the birthweight of the infant, independent of the
mother’s own birthweight and adult height (Martin, Smith, Frankel, & Gunnell, 2004), implying that this is not the case and thereby extending interest
to health later in the maternal life course.
Health in early maternal life: accumulated health (3)
The theory of accumulated health states that health insults add up over the
life course, to gradually increase risks of ill health (Montgomery, Bartley,
Cook, & Wadsworth, 1996). This would mean that ill health in childhood
would be detrimental to adult health status. It has been suggested that ill
health in adults with lower education can be partly explained by childhood
health in a cumulative fashion (Kuh & Ben-Shlomo, 2004). One study
showed that 5-10% of the higher risk for individuals with a low education
was explained by childhood illness when adjustment was made for socioeconomic group (van de Mheen, Stronks, Looman, & Mackenbach, 1998b).
This would be a biological pathway supporting the theory of accumulative health. The concept of something being cumulative is also used in another sense, referring to cumulative disadvantage over the life course by way
of social pathways of continuous disadvantage (Graham, 2001; Kuh, Power,
Blane et al., 2004). This theory would include (1), (7) and probably (8) and
(9).
Socio-economic status in early maternal life (4)
To give an example of how social difficulties in childhood have been shown
to influence adult health, results from a Swedish study with a representative
sample of the Swedish population are presented (Lundberg, 1993). Dissention in the family and economic difficulties were among other dimensions
associated with a higher risk for adult ill health. Even when both these items
were in the same model, they were significantly associated with general
physical health, aches and pains, as well as mental health. Thus, to the extent
that economic difficulties are more common in families with lower socioeconomic status, they may mediate socio-economic inequalities in adult
health.
33
Health selection from childhood to adult socio-economic status (5)
Because of the importance of childhood for the pathway into adult socioeconomic status, it is worth considering the theory of health selection in two
steps. The selection from childhood to adulthood is sometimes referred to as
intergenerational (see for example Lundberg, 1991b), since the measurement
of childhood socio-economic status is usually based on parental indicators.
Intra-generational selection, which exists within the adult life, will be considered in the next section (6).
This theory of health selection concerns the influence of childhood health
on adult socio-economic status: individuals with better health would arrive at
a higher socio-economic status, whereby health is causing the difference
rather than the social factors being causal. There is evidence that physical
health does go hand in hand with upward social mobility (Blane, Smith, &
Hart, 1999; Illsley, 1955). For example, Swedish data have shown that taller
people are more upwardly mobile than shorter people (Nyström Peck, 1992).
While this process seems stable, it must be considered how this affects the
health gradient to assess whether it actually produces health differences. It
has in fact been suggested that this health selection is indirect, in that it is
probably other factors such as family values that are behind both the better
health and the upward mobility (West, 1991), which follows from the influence of childhood socio-economic status on that in adulthood, presented
above (1). It has been pointed out that when health selection has been considered important, it has been with reference to younger ages, when social
mobility is higher, and that the influence of this on the health gradient in
adult health ought to be diminished over time (Fox, Goldblatt, & Jones,
1985). Social mobility in Sweden has been shown to be quite high before the
age of 35 (Jonsson & Erikson, 1997), and this would include the majority of
women giving birth.
Health selection in adult life (6)
The original theory of health selection is perhaps more applicable to selection from childhood, but this distinction is not necessarily always made.
Since the original Black Report (Black, 1980), the importance of health selection for inequalities in adult health has generally been considered small,
and rather as decreasing than increasing differences between social classes
(Blane, Harding, & Rosato, 1999; Blane, Smith, & Hart, 1999; Rahkonen,
Arber, & Lahelma, 1997). This is because the health of the lower childhood
socio-economic status individuals who are upwardly mobile, has been found
to be better than that of those who remain in a position (or are downwardly
mobile), but worse than that of the higher socio-economic status individuals
they replace (Blane, Harding, & Rosato, 1999).
34
In addition to the original health selection theory, there are other selection
factors more limited to adulthood, and these seem to more undoubtedly decrease the health gradient. According to “the healthy worker effect”, those
able to attain and maintain a job, are healthier than those who don’t (Dahl,
1993). Furthermore, a selection out of physically demanding jobs, both out
of the labour market and into lower non-manual jobs that are less physically
demanding, has been shown (Östlin, 1988).
It seems relevant here to provide an example of how the proposed framework could be extended and could benefit from separating the socioeconomic indicators. Selection out of physically demanding jobs might not
increase differences between manual and non-manual classes. However,
effects on income may be considerable and would in thus increase health
inequalities over income groups.
Adult socio-economic status and adult health: living conditions (7)
A definition of level of living often used in Swedish research on welfare is
given by Fritzell and Lundberg (2006) as the possibility for people to control
their own lives with the help of their own resources. They mention for instance knowledge, economic resources and physical energy. In a given context, the individual may use these to exert influence on her living conditions
(Erikson & Åberg, 1987). Here, material living standard and working conditions will be considered.
Material factors such as income or financial difficulties are often found to
be of importance for social class inequalities in adult health (Laaksonen,
Roos, Rahkonen, Martikainen, & Lahelma, 2005). Some of the effect of the
material factors has been found to be mediated via psychosocial pathways or
health-related behaviours (van Oort, van Lenthe, & Mackenbach, 2005). It
should be noted that material circumstances, or material living standards,
refer not only to deprivation or poverty. Swedish research has claimed that
both absolute and relative income are associated with health outcomes, and
that the relations are independent of each other and might be of varying importance for individuals depending on their position in the income distribution (Åberg Yngwe, 2005).
Physical working conditions have been shown to be important for adult
health, through repetitive strain injuries, accidents and all kinds of hazardous
exposures, and have been shown to be the single most important explanation
for socio-economic inequalities in health in Sweden (Lundberg, 1991a).
Likewise, more recent studies empirically explaining socio-economic inequalities in self-rated health with working conditions found physical demands to be the most important explanatory factor (Borg & Kristensen,
2000; Schrijvers, van de Mheen, Stronks, & Mackenbach, 1998).
Working conditions are likely to contribute to socio-economic inequalities in birth outcome as well. Occupational research has shown that maternal
35
exposure to various work environments may affect time to pregnancy
(Rylander, Axmon, Toren, & Albin, 2002), birthweight (Farrow, Shea, &
Little, 1998), gestational age (Sanjose, Roman, & Beral, 1991) and foetal
growth (Seidler, Raum, Arabin, Hellenbrand, Walter, & Schwartz, 1999). A
Swedish study showed differences in low birthweight rates for some occupations in 1981, and possibly in perinatal deaths and malformations (Ericson,
Eriksson, Källén, & Zetterström, 1987). In a meta-analysis it was concluded
that physically demanding work was the factor most clearly associated with
preterm birth and small-for-gestational-age (Mozurkewich, Luke, Avni, &
Wolf, 2000). Studies III and IV will explore the mediating role of working
conditions in class inequalities in birth outcome.
Adult socio-economic status and adult health: lifestyle and
behaviours (7)
Further from the social structure and closer to the individual is health-related
behaviour (Lundberg, 2003). It is well known that dietary and exercise habits, as well as smoking and the use of alcohol, are not only related to health
but also vary by socio-economic status so that in general (but not always) the
lower the socio-economic status, the more the likelihood of behaviours detrimental to health. These behaviours are established in childhood, and childhood socio-economic status has sometimes been shown to have an independent effect on adult health behaviours (van de Mheen, Stronks, Looman,
& Mackenbach, 1998a), and sometimes to change (diet) with change of
socio-economic status in adult life (Mishra, Prynne, Paul, Greenberg, & Bolton-Smith, 2004).
Whereas moderate intake of alcohol has been difficult to relate to the birth
outcome of interest here (Henderson, Gray, & Brocklehurst, 2007), there are
indications that this may influence cognitive outcomes later in life (Huizink
& Mulder, 2006), ADHD for instance. The issue of alcohol consumption
during pregnancy should therefore not be disregarded, especially since the
increased alcohol consumption in Swedish women in childbearing ages
seems to have increased (Bergman & Källmén, 2003), and also seems to be
higher among pregnant women that previously thought (Göransson, Magnusson, Bergman, Rydberg, & Heilig, 2003).
Smoking has been shown to both decrease birthweight (Wen, Goldenberg,
Cutter, Hoffman, Cliver, Davis et al., 1990) and increase the risk of mortality
(Wilcox, 1993). From the early 1970s and onwards, smoking has been more
prevalent in women with lower socio-economic status, and over time it has
decreased much less in these groups than in others (National Board of Health
and Welfare, 1997). This makes smoking an important mediating factor in
socio-economic inequalities in birth outcome. As mentioned above, smoking
36
has been the most commonly used empirical explanation for socio-economic
inequalities in birth outcome in Nordic studies.
From a sociological perspective, two important issues regarding smoking
need to be addressed, both relating to the determinants of smoking rather
than the direct detrimental effect it has on maternal health and birth outcome.
Firstly, the majority of women with lower socio-economic status (for example those in the manual classes) do in fact abstain from smoking and the
choice of taking up or not quitting smoking is unlikely to be at random. It
has been argued that smoking behaviour starts in adolescence, where it can
be considered a marker of those experiencing less academic success and not
aspiring to further education (Koivusilta, Rimpela, Rimpela, & Vikat, 2001).
Secondly, smoking is known to increase with experienced stress, from working conditions, for example (Johansson, Johnson, & Hall, 1991). Adult life
also presents socio-economic differences relating to stress (see also the section on stress below). Individuals with lower socio-economic status experience more stressful events and have worse coping strategies (Bosma, van de
Mheen, & Mackenbach, 1999; Kristenson, Eriksen, Sluiter, Starke, & Ursin,
2004; Stronks, van de Mheen, Looman, & Mackenbach, 1998). In that it is
so tied to the social structure, smoking can not be seen exclusively as an
individual choice. If we look at women of low education and compare those
who smoke to those who do not, the smokers are likely to be more disadvantaged. The significance of this, is that when controlling for smoking we
probably control for more than just the toxic effect of smoking, and might
even see reductions in health differences for outcomes where smoking is not
even on the causal pathway.
Health behaviours are sometimes equated with factors such as BMI. This
will here be regarded as a health indicator and discussed under (8). For reasons of simplicity, other issues of weight and nutrition are placed there as
well.
Direct references to maternal health indicators (8)
In the illustrative model, arrow (8) represents all the ways maternal health or
body can have an influence on infant health, in other words all influence that
can be considered to be mediated by the maternal body. It has been claimed
that “To some extent, poor pregnancy outcomes are a manifestation of
chronic disease processes that are already underway in young women”
(Rich-Edwards & Grizzard, 2005, p 532). Here I will present the association
between some maternal health indicators and birth outcomes. In a later section the health transfer between mother and infant will be attended to.
In a comparatively rich country such as Sweden with high living standards, restricted caloric intake is seldom due to financial restraints. Maternal
nutritional status might nevertheless be interesting. For example, women
with a lower BMI at the beginning of pregnancy have increased risks of giv37
ing birth to SGA (Clausson, Cnattingius, & Axelsson, 1998) and preterm
infants (Ancel, Saurel-Cubizolles, Carlo Di Renzo et al., 1999). This could
be due to either constitutional differences or caloric intake. The increasing
rate of anorexia nervosa is not likely to influence socio-economic inequalities in birth outcome at a population level – firstly, because reproductive
functions are repressed early on in the disease (Wolfe, 2005), and risks for
adverse birth outcome to women with a history of anorexia nervosa have not
been greatly elevated (Ekeus, Lindberg, Lindblad, & Hjern, 2006); secondly,
because anorexia nervosa is presently more common among women from
higher socio-economic status families (Nevonen & Norring, 2004).
Higher BMI is considered a risk factor with regard to birth outcomes, for
example preterm and especially very preterm births (Ancel, SaurelCubizolles, Carlo Di Renzo et al., 1999). Obesity is linked to pregnancy
disorders and an increased risk for several birth outcomes (King, 2006; Yu,
Teoh, & Robinson, 2006), and hypertension and pre-eclampsia are especially
associated with higher rates of very preterm SGA (Clausson, Cnattingius, &
Axelsson, 1998). Higher BMI is more common in women with lower socioeconomic status (Ball, Mishra, & Crawford, 2002; Rahkonen, Lundberg,
Lahelma, & Huuhka, 1998), and could be a mediating factor in socioeconomic inequalities in birth outcome.
Of more recent interest is the possible impact of micronutrients on optimal birth outcome, whereby intake may vary between socio-economic
groups, but evidence is still week concerning the extent to which micronutrients might have an impact on birth outcome (Kramer, Goulet, Lydon, Seguin, McNamara, Dassa et al., 2001). It might also be relevant to stress here
that maternal and foetal nutrition are not one and the same. Because the foetus receives nutrition from the placenta, foetal nutrition may be problematic
even though maternal nutrition is adequate (Harding, 2001).
Socio-economic factors after birth (9)
All ways in which parental socio-economic status may influence infant
health not mediated by the maternal body are considered under this heading.
Theoretical possibilities of mediators exemplified here are sleeping position
and breastfeeding. An infant sleeping on its stomach has an increased risk of
death classified as Sudden Infant Death Syndrome, and if the propensity to
make sure infants do not sleep in this position is higher in higher socioeconomic groups this could mediate inequalities. To the extent that breastfeeding is viewed as a possible choice for all mothers, this is another theoretical mechanism. If breastfeeding in Sweden today improves infant health,
and is carried out to a higher extent in higher socio-economic groups which
is suggested (Flacking, Hedberg Nyqvist, & Ewald, 2007), this would mediate inequalities. However, this can not be considered to be entirely not inde-
38
pendent of maternal health in the respect that the infant feeds direct from the
maternal body.
Arrows (8) and (9) will be included in all studies in this thesis, since together they represent the two possible pathways through which infant health
can be influenced by maternal socio-economic status.
Access to care
Access to care has been included to illustrate how something may be both
interesting for the influence of maternal health and applicable to socioeconomic environment after birth.
According to the theory of different access to health services or the “inverse care law” (Hart, 1971), individuals with a lower socio-economic status
have less access due to more restricted financial resources, or experience of
less respectful treatment, or a systematic discrepancy in judgment of symptoms. The health care system in Sweden is constructed so as to be equally
accessible by all. The patient’s fee is low and more advanced medical care is
virtually free of charge. In spite of this, however, there seems to be socioeconomic differences in use of medical care.
In a review of several Swedish studies, it was found that people with
lower socio-economic status in comparison to those with higher status made
fewer primary care visits (“öppenvårdsbesök”) in relation to their health
status. Less research seems focused on hospital visits, but there are indications that individuals with higher income were more likely to receive hospital care (Burström & Foree, 2001).
Therefore, there might be differences in socio-economic status in received
care. However, it seems unlikely that there are any major inequalities in maternal care during pregnancy, which is used by nearly 100% of pregnant
women (National Board of Health and Welfare, 2005), and should cover the
entire socio-economic scale. Neonatal care in Sweden is internationally outstanding, and early infant care is clearly initiated at the hospital. There may
be socio-economic differences in how the health care system is approached
after the hospital stay, but seemingly, no study of maternal or infant care was
found.
Stress – an important link?
This paragraph will describe an interesting branch of research that seems
involved in most of the boxes in the illustrative model. It is presented here
not only because maternal stress seems relevant to birth outcome, but it also
seems especially important for the link between maternal and infant health.
Pathways from stress to adverse outcomes have been conceptualised as a
disturbed balance in the body. The body was made to handle stress, but primarily acute stress, in situations that must be dealt with immediately
39
(Sapolsky, 1998). When the body has a constant stress alert turned on, less
energy is invested in the immune system and repairing processes in the body.
This has been described as stress response turning into an allostatic load,
which is detrimental to several dimensions of health, such as cardiovascular
health and the immune system, increasing one’s general vulnerability to all
health problems (McEwen & Norton Lasley, 2004).
The relation between stress and socio-economic status seems twofold.
Firstly, it has been shown that stressors are much more prevalent in the lives
of families with a lower socio-economic status, for example those experiencing hardship or economic difficulties. Secondly, response to stressors is affected by how the mind interprets the stressor, and it has been claimed that
psychological coping abilities with stressors are better in higher socioeconomic status families (Kristenson, Eriksen, Sluiter et al., 2004; Stronks,
van de Mheen, Looman et al., 1998). One study showed that part of the association between childhood class and adult health was explained by variation in coping strategies (Bosma, van de Mheen, & Mackenbach, 1999).
However, to this it should be added that besides the psychological dimension, material and possibly other resources are likely to be more available in
these families, which also ought to have a very practical impact on available
strategies for dealing with stressful situations.
Reviews seem to agree that the link between stress and preterm birth is
well established (Gennaro & Hennessey, 2003; Hobel & Culhane, 2003;
Hogue, Hoffman, & Hatch, 2001), but there seems to be less support for the
association with foetal growth (Hobel & Culhane, 2003). However, if stress
shortens length of gestation, it will indirectly decrease birthweight and increase mortality risk. Long-term stress has been specifically proposed as a
pathway for socio-economic inequalities in birth outcome (Kramer, Goulet,
Lydon et al., 2001; Morello-Frosch & Shenassa, 2006; Rich-Edwards &
Grizzard, 2005).
In addition to the direct effect it seems to have, stress is linked to adverse
health behaviours such as smoking as mentioned earlier. The importance of
this mechanism may vary with the general level of inequality in a society
(see “Ecological factors”).
Transfer of health between generations
The reproduction of socio-economic status inequalities likely involves an
intricate pattern of biological and social pathways. In the illustrative model,
maternal childhood is viewed from a life-course perspective. An alternative
perspective is to think of the mother’s childhood as having been influenced
by her parents, her infant’s grandparents. Socio-economic inequalities in
birth outcome are of further interest and importance because infant health
constitutes a point of health transfer between generations. In the creation of
40
a new individual, a starting point of health potential (or capital) is received
by this individual, and this is not independent of parental socio-economic
status. In this discussion, the concept of epigenetic inheritance is useful. The
phenomenon of epigenetic inheritance has long been acknowledged in cancer research, for example, but it was only quite recently that strands of research from several disciplines were brought together, realising they were
working around a new central concept (Pray, 2004). Epigenetic inheritance
comprises, for example, the on or off switch of genes, through the inheritance of the amount of molecular groups on the histon; a broader definition
includes the setting of hormonal systems, the HPA axis being the most researched (Jablonka, 2004). The term “epi” means “beyond” or “besides,
implying that besides the genetic code, influence of parental environment
with its limitations or abundances can be inherited by the next generation.
That the environment may influence across generations is a possible way
to interpret changes in population health that have occurred much faster than
a change of the genetic code – the adaptation process of which takes much
more than just a few generations to evolve. For example, populations in developed countries such as Sweden have become taller over the past century.
This can not be a genetic change, but is believed to have been brought about
by an improvement in living standards. Following this, it may very well be
the case that children in families with better living standards – generally
approximated by higher socio-economic status – have had the possibility to
fulfil their genetic potential to a greater degree than other children have.
Short adult height, then, has been claimed to be a reflection of growth during
the childhood period, for example (Peck & Lundberg, 1995). A later study
was able to link the birthweight of infants to their mothers’ height in childhood (as a marker of social and nutritional exposure) independent of maternal childhood socio-economic status or adult height (Martin, Smith, Frankel
et al., 2004). Thus, when a study showing that shorter mothers with a manual
background in comparison to those with a non-manual background have
higher proportions of disadvantaged birth outcome, this could be due to their
having reached a smaller proportion of their genetic potential (Krieger,
Chen, & Selby, 2001).
I would suggest that this transfer between generations, the process of
which could be argued to end with the cutting of the umbilical cord, should
be conceptually separated from what is often referred to as reproduction of
health. The concept of reproduction could rather be reserved to signify processes by which human agents become socialised into behaviours and predispositions (1).
This section concludes the outline of an explanatory framework, and is
followed by a presentation of the specific aims and the studies of this thesis.
41
Studies
As previously stated, the general aim of this thesis is to contribute to the
understanding of the association between mother’s socio-economic status
and the health-related birth outcome of their infants. This is accomplished
through a focus on maternal education and class, studying a range of birth
outcomes. Below is a brief presentation of Studies I-IV, summarizing the
specific aims, data materials, results and interpretations of these. The studies
in their entirety are found after the introductory chapters.
Specific aims
•
To explore the association between maternal education and infant health
in Sweden (Study I)
•
To investigate the combined influence of maternal childhood and adult
social class on infant health (Study II)
•
To investigate how maternal childhood and adult social class influence
birth outcome closer compared to further from the birth of the infant
(Study II)
•
To explore the extent to which maternal working conditions might contribute to social class inequalities in birth outcome (Studies III & IV)
Data
MEGA Database, Studies I-III
This database was constructed specifically to enable the simultaneous investigation of maternal childhood and adult class in relation to infant health.
Registers used for the database are the Swedish Censuses (1960, 1970, 1980,
1985), the Education Register (1990) and the Medical Birth Register (197390). In order to collect maternal childhood class, women’s birth years had to
42
be 1946-60, resulting in registration of parental class in the 1960 census. The
population is thereby defined as all births 1973-90 to women born 1946-60
partaking in all the censuses 1960, 1970 and 1980. All infant deaths to this
population were used as numerators. Of the women in the population, 10%
were randomly selected as denominators (and multiplied by 10). The random
collection of 10% of women was dictated by Statistics Sweden, in order to
protect the integrity of individuals. Our data were all anonymous, without
personal identification numbers.
The MEGA Database was compared to all single births in Sweden for the
period 1973-90 from the Social Mobility Database (see next section). The
MEGA material was found to have slightly lower very and low birthweight
rates, but the same neonatal mortality rate. Maternal ages were slightly lower
in MEGA than in the reference database, and the proportion of births at the
beginning and end of the period was smaller. Due to the inclusion criteria of
1960, no immigrants arriving after this year are included in the MEGA database. This might contribute to the differences, although results from previous
research suggest that this contribution may be small (Koupilova, Vågerö,
Leon et al., 1998; Rasmussen, Oldenburg, Ericson, & Gunnarskog, 1995).
MEGA
Proportion of births (%)
60
SocMob
50
40
30
20
10
1973-78
1979-84
<2
0
20
-2
4
25
-2
9
30
-3
4
35
-3
9
>3
9
<2
0
20
-2
4
25
-2
9
30
-3
4
35
-3
9
>3
9
<2
0
20
-2
4
25
-2
9
30
-3
4
35
-3
9
>3
9
0
1985-90
Figure 2. Distribution of births over maternal age groups at three time periods, for
the MEGA and the Social Mobility Databases
43
Social Mobility Database, Study IV
This database is a combination of selected variables from many registers. In
this thesis, information from the Swedish 1980 and 1990 censuses were
used, along with data from the Medical Birth Register (1980-2002) and the
2002 Educational Register. These registers were used in their entirety,
whereby no selection was introduced.
Completeness and quality of registers
The Medical Birth Registry in Sweden started in 1973, and the number of
registered parameters has since been expanded. Several evaluations have
been made, which suggests that the general quality of the data is high
(Cnattingius, Ericson, Gunnarskog, & Källén, 1990; Källén & Källén, 2002).
The later, more thorough, evaluation has also acknowledged the fact that not
all variables are registered with the same precision. In this thesis no analyses
have been made with variables considered to be doubtful.
The Educational Register (Svensk utbildningsnomenklatur) is also of high
quality, updated yearly and with missing data below 2% (for 16-65-yearolds) (Statistics Sweden, 1993), but misclassifications of about 15%
(Statistics Sweden, 2006). For women, however, only about 1% is missing,
and data on people aged below 45 years, which is the group studied in this
thesis, is more reliable than for older groups (ibid).
The Swedish censuses are considered to be of very high quality, with high
accuracy and completeness.
Results and interpretations
Study I: Maternal education
Specific aims: To explore the association between maternal education and
infant health. To explore changes in the relation over time. To explore the
possible presence of the low birthweight paradox.
Results: We found that infants of women with less than three years of secondary school in comparison with those born to women with the highest (university or college) education had significantly higher risk of low birthweight,
and neonatal as well as postneonatal mortality. The excess risk for the group
with lowest in comparison with the highest education was 30% for infant
mortality and 67% for low birthweight when all years were taken toghether.
Over time, differences between infants of women with the lowest (up to
compulsory) and highest education increased for all outcomes. This devel44
opment brings about the so-called low birthweight paradox (see “Discussion” for an explanation).
Interpretation: With the decreasing proportion of women only achieving low
education, the group becomes increasingly composed of generally disadvantaged women; this is likely what is reflected in the increasing differences in
birth outcome. The low birthweight paradox suggests that the distribution of
determinants for low birthweight infants differs for women with low and
high education.
Study II: Maternal childhood and adult social class
Specific aims: To investigate the combined influence of childhood and adult
social class on infant health. To explore whether the influence of childhood
class is independent of adult class. To compare the influence of class on
birth outcome closer to and further from time of birth.
Results: Maternal childhood class was associated with both low birthweight
and infant mortality. A manual background, in comparison to a non-manual
background, implied between a 12% and 17% higher risk of low birthweight,
neonatal or postneonatal mortality. The increased risk of manual class was
higher for adult class, up to around 60% for some classes. Manual workers in
the manufacturing sector had the highest risks, irrespective of level of qualification. When adult class was included as well, an independent effect of
childhood class remained for low birthweight and neonatal, but not postneonatal, mortality. A manual childhood class increased the risk for these adverse outcomes by about 10%. The odds ratios were higher for outcomes
closer to birth. The influence of maternal adult class was stronger than that
of maternal childhood class.
Interpretation: Because the influence of childhood class is independent of
adult class for the outcomes closest to birth, we interpret this as support for a
biological pathway via maternal health. That is, something in the maternal
childhood has an influence on maternal biology, which in turn affects infant
health. Theoretically, maternal biology might be influenced in childhood or
later. There could be something in the mother’s childhood circumstances that
have a lingering effect on her reproductive system, or her childhood might
predispose her to behaviour and attitudes that remain in adulthood, for example a propensity to smoke or not quit smoking during pregnancy.
45
Study III: Maternal working conditions and infant mortality
Specific aims: To explore the relation between working conditions and infant
mortality. To estimate the extent to which working conditions might contribute to social class inequalities in infant mortality.
Results: Lower job control and higher physical demands were associated
with a higher risk for infant mortality. Results for job hazards were ambiguous. No association was found between psychological demands or social
support and infant mortality. Three of four classes of manual workers, as
well as self-employed, had a significantly (or borderline significant) higher
risk for infant mortality than did the reference group, higher non-manual
workers. This was reduced by 28-58% when job control, physical demands
and job hazards were simultaneously controlled for, rendering all class differences statistically insignificant. Job control had the strongest association
with infant mortality, and was the largest single explanatory dimension of
working conditions.
Interpretation: Social class inequalities in infant mortality are influenced by
maternal working conditions. There is good reason to further explore working conditions as contributors to socio-economic inequalities in birth outcome.
Study IV: Maternal working conditions and birth outcome
Specific aims: To explore the relation between psychological and physical
working conditions and a large number of birth outcomes. To estimate the
extent to which these factors might contribute to class inequalities in low and
very low birthweight, small-for-gestational-age, as well as very and extremely preterm births.
Results: The lowest level of job control was associated with a higher risk for
eight of nine adverse birth outcomes, and for five of these there was a risk
gradient over levels of job control. Higher levels of physical demands and
job hazards were associated with higher risks for adverse growth and gestational outcomes. There was only one significant association between physical demands and (neonatal) mortality. For most adverse birth outcomes, all
manuals and lower non-manuals had significantly higher risks than did the
reference category, middle non-manuals. Higher non-manuals had lower
odds for all outcomes (significant for two), and self-employed had higher
odds ratios for all outcomes (significant for three). The three dimensions of
working conditions contributed to different degrees to the different birth
outcomes. Job control explained a considerable proportion of all adverse
birth outcomes, and for low birthweight, SGA, all preterm and very preterm
46
birth this was still significant when the other dimensions were in the same
model. In spite of its contribution, job control was not significant in any
model for very low birthweight or extremely preterm birth. Job hazards contributed mostly to very low birthweight and extremely preterm birth; it was
also significant when the other working conditions were in the same model.
Physical demands explained a considerable proportion of low birthweight as
well as all preterm and extremely preterm births, but were only significant in
relation to the first two outcomes.
Interpretation: The dimensions of working conditions showed disparate
associations with the birth outcomes, indicating a high complexity in the
relations. An important issue is the timing of present legislation. According
to our results, measures need to be taken earlier on in pregnancy to avoid
adverse influence of working conditions, which result in socio-economic
inequalities in birth outcome.
47
Discussion
In the studies of this thesis, I have described and analysed the relation between two maternal socio-economic indicators, education and social class,
and a range of birth outcome. All studies find socio-economic inequalities in
all studied birth outcomes, contradicting some earlier statements on infant
health in Sweden (Black, 1980; Köhler, 1991). The general approach of this
thesis has been exploratory, because while the basic association between
socio-economic indicators and some birth outcomes today is well described
in the Nordic countries, as shown above, empirical attempts to explain this in
any depth have been more rare. One of the reasons for this could be that an
explicit explanatory framework has been missing.
The framework presented here may serve as a beginning of a discussion,
and the most important aspect in this is probably the importance of maternal
health (potential or capital). The proposed framework represents a sociological interpretation of the field of socio-economic inequalities in birth outcome. This framework has developed during the work on this thesis, and it is
from here that the studies constituting the thesis have emerged: Firstly, the
exploration of the low birthweight paradox and the way to study and interpret the influence of maternal socio-economic status in childhood; Secondly,
the extension of infant health outcomes from a focus on infant mortality and
birthweight to the investigation of a larger range of birth outcomes.
Although the general approach in the work on this thesis has been exploratory, I have tried to specify certain aspects of the association between
maternal socio-economic status and birth outcome. In the exploration of the
low birthweight paradox in Study I, with regard to larger maternal educational inequalities, I will argue that this knowledge is important for the interpretation of differences in birth outcome. In Study II a maternal life-course
perspective is introduced, whereby adult class is found to be of greater importance for infant health. For the outcomes closer to birth, however, there is
an independent influence of childhood, implying that this association might
be primarily mediated through the maternal body. As an attempt to initiate
the exploration of mechanisms between adult socio-economic status and
birth outcomes, working conditions were investigated in relation to social
class in Studies III and IV. There are indications in Study IV that some aspects of work may be detrimental to infant health, and that this is important
earlier in pregnancy than what is presently stated in health policy and legislation in Sweden.
48
Socio-economic status, individuals and households
An important issue in social research is how we choose socio-economic indicators and what we believe they entail, not only in relation to whether social class should be household or individual-based, as discussed above.
Strong arguments have been made against using explanatory power as a
main principle for choosing socio-economic indicator, for example because
this would mean that the measure is a composite one (Erikson, 2006). A
more pragmatic way could be to explicitly consider what we believe the
measure already “contains”, or is associated with, a question that arises in
the study of childhood class (Study II). Inequality seemed stronger for postneonatal than neonatal mortality when only adult class was used. When both
childhood and adult class were in the model, inequality seemed higher for
neonatal mortality. Thus, any indicator that is associated to a higher degree
with childhood class might result in higher inequality for neonatal than postneonatal mortality. In other words, it matters what indicator of socioeconomic status we use. It has even been claimed that standard indicators of
socio-economic status do not have the same meaning in all social groups
(Braveman, Cubbin, Egerter, Chideya, Marchi, Metzler et al., 2005), and this
discussion is not likely to end there.
In order to maintain a focus in this thesis, it has been limited to maternal
socio-economic indicators. However, there is no reason to believe that socioeconomic influence on birth outcome is limited to maternal indicators. It has
been shown, for example, that maternal and paternal education are independently associated with infant mortality (Arntzen, Samuelsen, Bakketeig,
& Stoltenberg, 2004). Including maternal partner opens up many new aspects to study. Married individuals and cohabitants generally have better
health outcomes than do single adults, and there are indications that this is
the case for infant health as well (Arntzen, Moum, Magnus et al., 1996b;
Leon, Vågerö, & Olausson, 1992). As mentioned previously, material living
standards is probably better in a household with two incomes. It has also
been claimed that class-mixed and homogenous couples are dissimilar with
regard to behaviour, for example (Leiulfsrud & Woodward, 1987); women’s
health behaviour has in fact been shown to be influenced by the social class
of her partner (Bartley, Martikainen, Shipley, & Marmot, 2004). Not all aspects of having a partner are positive, unfortunately. Difficult issues exist
concerning parenting with regard to violence by fathers, regarding both children and their mothers (Eriksson, 2004). It is suggested that marital violence
increases the risk of preterm birth (Moutquin, 2003) and low birthweight
(Bullock & McFarlane, 1989), and since this is suggested to be more common in lower socio-economic groups (Gazmararian, Adams, Saltzman,
Johnson, Bruce, Marks et al., 1995), this could possibly be a contributor to
inequalities in birth outcome. Another area of potential interest is secular
changes in paternal care of infants: if paternal status was more important 50
49
years ago, perhaps for material reasons, is it now more important for reasons
connected to fathers’ increased care for and participation in the lives of their
children?
Confounders and mediators
The relative importance of specific mediators is likely to vary with the chosen indicator of socio-economic status and the specific birth outcome. Maternal smoking, for example, shows a steeper gradient across educational
categories than social classes (Nordström & Cnattingius, 1996), and its detrimental effect on foetal growth seems larger than that on gestational age
(Kramer, Goulet, Lydon et al., 2001). Therefore, it will probably explain
more of the educational gradient in foetal growth than of the class gradient in
preterm birth.
Maternal height is sometimes adjusted for in analyses of birth outcomes.
Taller mothers, for example, have a smaller proportion of SGA (Clausson,
Cnattingius, & Axelsson, 1998). However, achieved maternal height can be
viewed as a composite measure of the outcome of social circumstances,
achieved growth in foetal life, infancy childhood and adolescence. While
this measures health status or health potential, it is also associated with social class. If maternal height is adjusted for in an analysis between SES and
birth outcome, it may be viewed as a pathway rather than a confounder
(Morton, 2004).
The explanatory factors from the Nordic studies referred to above are
quite similar to those suggested already 60 years ago by British researchers,
who concluded that no main explanatory factors had been identified (Morris
& Heady, 1955). Future research could empirically try to estimate the relative importance of material living conditions and working conditions, for
example. This should perhaps start with a discussion of what is actually implied by “living conditions”, a somewhat all-inclusive concept sometimes
drifting towards a measure focused almost exclusively on material living
conditions (see for example how "shared conditions" on page 502 becomes
"standard of living" on page 508 in Erikson, 1984a).
Perhaps women’s working conditions as a determinant of birth outcomes
have been underestimated in relation to material living conditions. Earlier,
the relation between maternal working conditions and infant health have
been thought to be generally limited to risks of mutagen and teratogenic
effects during pregnancy (Arbetsmiljökommissionen, 1989). When the link
between maternal and infant health has been discussed previously, access to
general health care for women has been mentioned (Wise, 2003). However,
general health is not only a function of high-quality health care. If general
working conditions in a country affect women’s health, and this is important
for birth outcome as has been suggested (Morris & Heady, 1955; Rich50
Edwards & Grizzard, 2005), this may influence the level of adverse birth
outcomes in a country. Especially so since, for example, our results indicate
that this is important early in pregnancy, when policies to protect infant
health are not applicable. In 1980 in Sweden, a law was passed under the
Work Environment Act granting women with heavy physical work demands
and specified kinds of chemical exposure a leave of absence from week 30
of pregnancy (National Social Insurance Board, 1988). In Study IV extremely preterm births (born before 28 weeks) were significantly associated
with both maternal class and working conditions, implying that work-related
policies on pregnancy need to be improved.
Future research may be directed towards working conditions on an aggregated level. Results from the Second European Survey indicate that physical
working conditions are harsher in southern Europe than in the Nordic countries, for example, which is likely to be due to the areas’ disparate positions
in the industrialisation process (Eklund, Englund, & Wikman, 2001), and
these countries’ generally have higher infant mortality, for example (see
Table 2). Recent studies have shown that the physical environment for
women in Sweden has actually deteriorated since 1981 with respect to ergonomic load (Hemström, Krantz, & Roos, 2006), and between 1996 and 1999
in respect to sickness benefits for work-related health problems (Vikenmark
& Andersson, 2002). It would also be of interest to study how this might
influence birth outcome.
The relevance of the low birthweight paradox
Measuring inequality in birth outcome over time can be problematic when
the level of adverse outcome decreases substantially, which was the case for
infant mortality during the 20th century. This is because if we use logistic
regression and obtain odds ratios, relative differences can become very large
when absolute levels are quite small. Presenting absolute differences has
thus been recommended as a solution (Lundberg, 2003), and provides good
illustration and may be accomplished, for example, by showing figures for
one parity and a specific maternal age group. However, drawing comparisons with odds ratios can produce interesting results as well; an example of
this is the low birthweight paradox.
In Study I education inequalities in infant mortality increased over the
study period, especially when comparing infants of the most educated
women to those of the least educated women. With the decreasing proportion of women who only achieve a low education, the group becomes increasingly composed of generally disadvantaged women; this is likely what
is reflected in the increasing inequalities in birth outcome. A similar explanation was offered for the educational difference in mortality in adult men
and women during the period 1986-2003 (Statistics Sweden, 2004). When
51
separating births into birthweight categories, however, it can be observed
that for infants with low birthweight the mortality risk is lower for those
born to women with a low education. This phenomenon is referred to as the
low birthweight paradox. When this was first found in relation to smoking
(low-birthweight infants born to smoking women had lower mortality) it was
promoted as an advantage for smokers (see Wilcox, 2001). Increasing
knowledge about the issue revealed that this was because smoking decreased
the birthweight of the infants, and although mortality increased as well, it did
not increase to the same level of an infant who weighed less due to causes
other than smoking. Therefore, the cause of the paradox is likely that causes
of low birthweight are differentially related to mortality risk, and that the
relative importance of these determinants varies across populations.
Without this knowledge we could wrongly assume that it is an advantage
for infants born to women with lower socio-economic status to have low
birthweight. Or, when only low birthweight infants are studied, it might be
concluded that there are no socio-economic inequalities – or that they are
reversed if the paradox appears. An extension of this research, attempting to
clarify what we find when comparing infants born to women with different
socio-economic status, could be to go beyond birthweight. As stated earlier,
birthweight is an imprecise endpoint. Preliminary analysis of the Social Mobility Database implies that there is a gestational age paradox (data not
shown), and there might also be a “foetal growth paradox”.
The dynamic character of health inequalities
Research has shown that both inequalities and their causes vary over time
and between countries, which can be illustrated with infant mortality, for
example (Newland, 1981). This is a dynamic association, so that to the same
extent that the significance of social class in a society changes, so can the
health consequences be expected to change (Leon & Walt, 1998). The dynamic character of the association is also evidence that health inequalities
can be reduced (Graham, 2001).
That the ability to maximise a pregnancy outcome may go back at least
one generation to when the mother herself was a foetus (Misra, Astone, &
Lynch, 2005) opens up a whole range of possible research perspectives.
There are indications in Study II that something in maternal childhood is
associated with her birth outcome in adult life. For low birthweight and neonatal mortality this was independent of adult social class, which implies that
the pathway is via maternal health. This would imply that childhood class is
manifested in the mother’s body, influencing the health of her infant. An
interesting question regarding maternal working conditions remains: to what
extent is the influence of maternal working conditions on infant health po-
52
tential mediated through the maternal health potential/body (8) and to what
extent does it influence birth outcome directly (9)?
With the development of the epigenetic concept, we can discuss biological heritage that is not genetically driven. For example, it was mentioned
above how population height has increased with population health, much
faster than genetic codes change with evolution. When this happens, there is
reason to believe that more advantageous groups in the population increase
their health before more disadvantageous groups do. For example, the low
level of infant mortality enjoyed by infants born to women with higher education in the early 1970’s was enjoyed by infants born to women with lower
education in the late 1980’s (data not shown). Thus, the health of the more
advantaged can be seen as goals that everyone has the possibility to reach.
This has been claimed to be a fundamental social cause of inequalities in
health, whereby resources are always greater for those with higher socioeconomic status, allowing them to improve their health first (Link & Phelan,
1996). However, while they seem to improve their health first, the theory is
probably a simplification as it does not distinguish between resources and
how this may influence socio-economic inequalities in specific health outcomes. The question of confounders or mediators is relevant here as well: In
relation to developmental origins of disease, social class differences are
sometimes presented as problematic because they are a confounder in this
process (Terry & Susser, 2001). In the framework of this thesis, developmental origins of disease are rather viewed as a possible mediator, a pathway
in the processes of inequality.
Children born to those who were malnourished or starved only 100 years
ago in Sweden probably did not achieve their full health potential during
their time in utero or early childhood. Today, an effect of this may still be
lingering in the infant health of their children. The extent to which this affects the socio-economic health gradient in birth outcome is obviously extremely difficult to analyse.
The co-evolution of health development and social development has an
impact across generations. A birth outcome can be seen as a point of health
transfer between generations. The new individual has not yet been socialised
into the complex social world we live in. Still, socio-economic differences
that have an influence on one generation of women are manifested as a biological starting point for their children. In Sweden and other countries with
very low infant mortality and smaller socio-economic inequality, infant mortality is probably not the most interesting outcome to pursue. The other birth
outcomes, such as gestational age and foetal growth, are of more interest
here. This is because while mortality is the most adverse outcome, the survivors will grow up and be affected by social circumstances in their adult
lives. Thus, the birth outcome of the survivors is perhaps of even more interest, in the complex web of health and social factors.
53
Some conclusions – relevance of the results
Socio-economic inequalities in birth outcome have been in focus in this thesis. Some conclusions can be drawn from this research:
•
Birth outcome is of special interest because it constitutes a point of
health transfer between maternal and infant health, and is heavily influenced by maternal social context.
•
We have shown that there were educational differences in birth outcome
in Sweden in the late 20th century, and that in comparisons of infants of
the most educated women to those of the least educated women, inequalities seem to increase.
•
The low birthweight paradox is of significance in the interpretation of
inequalities. Also, it suggests that the distribution of causes of low
birthweight have varying associations with infant mortality, and that the
proportion of these causes vary across educational groups.
•
To our knowledge, the simultaneous exploration of the influence of maternal childhood and adult social class has not been performed before.
Results indicate that a mother’s childhood social class affects her biology, so that her infant’s health is also influenced.
•
Working conditions have previously been used to explain social class
differences in adult health, but not in infant health. That maternal working conditions seem to influence social class inequalities in birth outcome makes this a possible area for policy change and reduction of inequality.
•
An important policy implication is that adjustment of working conditions is important much earlier in the pregnancy than what is presently
considered appropriate.
•
The co-development of health and social careers has an impact across
generations. Socio-economic differences that have influence on one generation of women are manifested as a biological starting point for their
children.
54
Acknowledgements
There are many people who deserve a thank you at the end of the work on
this thesis. My supervisor, Professor Denny Vågerö, not only suggested that
I become a PhD student, but has supported me throughout this time. We
have had many inspirational discussions, and while he has allowed me room
to develop my own way of thinking I always had to provide good arguments.
I have also had the privilege to work with associate professor Örjan Hemström. He is not only an incredibly supportive person, but I have also learned
from him to be more thorough and specific, and he has supplied me with a
way of thinking that has helped me understand abstract issues more concretely.
Besides these two people, I have had support and encouragement from many
people at CHESS - thank you all! I would especially like to thank Ilona
Koupil for showing interest in my work, and encouraging me to join the
UBCoS research group. For never-ending assistance in the final part of the
project, thank you Ylva Eriksson! And for support in the trials of being a
doctoral student, as well as more practical work-related discussions, thank
you Susanna Toivanen and Erik Malmstig. And thank you Judith Black and
especially Judith Rinker for patiently revising my English. I seem to
(vaguely!) remember a life before being a PhD-student, which seems even
further away in time since I also become a parent during this period. There
are two people without whom I would not have continued to study sociology, this strangely society-focussed subject so foreign to a former student of
microbiology. Thank you Maria Bagger-Sjöbäck, for giving the Department
of Sociology a friendly face. And to Nils-Magnus Björkman: thank you for
your devotion to your students!
I would like to end this section with a thank you to my friends Katarina
Graffman and Katarina Edin, for being there despite my focus on work and
family! Anders Rillbert and Yasmin Andersson – I hope we will succeed at
staying in contact for a very long time. My sisters, Karin Dahlén and Linda
Dahlén, I am so happy to have you! I also want to thank my parents, Ingalill
Dahlén and Arvid Sørensen, for their support over the years, and my brother
Owe Sørensen for managing to stay in contact even in difficult times.
Finally, thank you to my family, Line and Liv, and I am so sorry for having worked so much during the last months! Now I’ll be around more, and
prepare for the arrival of a fourth family member. All my love to you.
Marit Gisselmann, April 2007
55
This work was financially supported by the Swedish Council for Working
Life and Social Research, the Swedish Research Council and the Department
of Sociology at Stockholm University, which is gratefully acknowledged.
56
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