Socioeconomic status and risk of infant death. A population-based study of trends in Norway, 1967–1998

Annett Arntzen1, Sven Ove Samuelsen2, Leiv S Bakketeig3 and Camilla Stoltenberg4

1 Faculty of Social Science, Vestfold University College, P.O. Box 2243, N-3303 Tønsberg, Norway
2 Department of Mathematics, University of Oslo, P.O. Box 1053 Blindern, N-0316 Oslo, Norway
3 Institute of Public Health, University of Southern Denmark, Sdr. Boulevard 23A, DK-5000 Odense C, Denmark
4 Division of Epidemiology, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, N-0403 Oslo, Norway

Correspondence: Annett Arntzen, Faculty of Social Science, Vestfold University College, P.O. Box 2243, N-3303 Tønsberg, Norway. E-mail: Annett.Arntzen{at}hive.no


    Abstract
 Top
 Abstract
 Methods
 Results
 Discussion
 References
 
Background The aim of this study was to examine the association between socioeconomic status and risk of infant death in Norway from 1967 to 1998.

Methods Information from the Medical Birth Registry of Norway on all live births and infant deaths was linked to information from Statistics Norway on parents' education. There were 1 777 364 eligible live births and 15 517 infant deaths. Differences between education groups were estimated as risk differences, relative risks, population attributable fractions, and index of inequality ratios.

Results The risk of infant death decreased in all education groups, and the level of education increased over time. The trends differed for neonatal and postneonatal death. For neonatal death the risk difference between infants whose mothers had high and low education was reduced from 3.5/1000 in the 1970s to 0.9/1000 in the 1990s. The relative index of inequality (RII) for maternal education decreased from 1.72 to1.32. The proportion of neonatal deaths that could be attributed to <13 years of education decreased from 22.3 to 8.4. For postneonatal death the risk difference between infants whose mothers had high and low education increased from 0.7/1000 in the 1970s to 2.0/1000 in the 1990s. The RII for maternal education increased from 1.31 to 4.00. The population attributable fraction increased from 9.7 to 39.5.

Conclusions An inverse association between socioeconomic status and risk of postneonatal death persists, albeit there was a considerable reduction in risk between 1967 and 1998.


Keywords Infant death, neonatal death, postneonatal death, socioeconomic status, time-trends

Accepted 17 October 2003

Infant mortality is widely used as an indicator of public health, the quality of health services, distribution of wealth, and the general standard of living in a society.1,2 A consistent long-term investment in social welfare and health care has gone alongside a substantial reduction in infant mortality in Norway. The infant mortality rate decreased from 33.9 per 1000 live births in 1950 to 4.0 per 1000 in the year 2000.3

An inverse association between the risk of infant death and socioeconomic status (SES) has been demonstrated across countries,4,5 within countries,6,7 and between social factors.8,9 Today, the risk of infant death in Norway is low and so are social inequalities in infant mortality.10–12 However, recent studies suggest that despite a decrease in infant mortality, social inequalities persist and may even increase.13–18

We wanted to examine the association between infant mortality and SES, measured as parents' educational level, from 1967 to 1998. We analysed neonatal and postneonatal mortality separately to assess whether social differences in infant mortality were increasing, decreasing, or stable in the period from 1967 to 1998.


    Methods
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 Abstract
 Methods
 Results
 Discussion
 References
 
Study population
The Medical Birth Registry of Norway has recorded all births after 16 weeks of gestation since 1967; a total of 1 848 087 live births were included in the period under study (1967–1998). These registry data were linked to information from Statistics Norway on parental educational levels attained by 31 December 1998. There was missing information on maternal and/or paternal education for 70 723 infants. These births were excluded from the present analysis, yielding a study population of 1 777 364 live-born infants. The infants were allocated to three cohorts. The first cohort includes 762 716 live births in Norway from 1967 through 1979 (the 1970s), the second cohort includes 509 426 live births from 1980 through 1989 (the 1980s), and the third cohort includes 505 222 live births from 1990 through 1998 (the 1990s) (Table 1).


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Table 1 Characteristics of the study population. Maternal age, parity, plurality, marital status, and maternal and paternal education (number of years completed in 1998) for all live born infants in Norway, 1967–1998. Three birth cohorts: 1967–1979, 1980–1989, 1990–1998, number and per cent

 
Variables
Neonatal and postneonatal death were the outcome variables. There were 15 517 infant deaths (0–364 days); 10 434 of the infants died neonatally (0–27 days) and 5083 died in the postneonatal period (28–364 days). Neonatal and postneonatal mortality rates are expressed as death per 1000 live-born infants.

Maternal age, parity, plurality, marital status, and parents' education were chosen as independent variables. Mother's age at delivery was categorized as <20 years, 20–34 years, and >=35 years. Parity was categorized as first-born and later-born infants. Plurality was divided into single and multiple births. Marital status was categorized as married/cohabitating and unmarried. The Medical Birth Registry has recorded ‘cohabitation’ since June 1982. In the 1980s there were 51 679 cohabiting mothers (10.2%) and 189 241 (37.6%) were recorded as ‘cohabiting’ in the 1990s. Preliminary analysis showed no differences between the groups ‘married’ and ‘cohabiting’ mothers according to infant death, thus the dichotomy of marital status: ‘married/cohabiting’ and ‘unmarried‘. The unmarried group includes predominantly women who have never been married, in addition to separated, divorced and widowed women (17 176 mothers, 1.0%).

For 120 869 (6.8%) births, information about the father was not reported in the Medical Birth Registry. Unmarried mothers are over-represented in the group with unreported fathers (89.3%). Information on mothers' and fathers' educational levels was obtained from the national education registry in Statistics Norway. Educational level is recorded as the number of years of education attained by 31 December 1998. The highest examination passed, according to the International Standard Classification of Education (ISCED) was categorized into three levels. More than 12 years of education is categorized as high, 10–12 years as medium, and <10 years as low education (Tables 2GoGoGoGo7). Education was not reported for 42 777 (2.3%) of the fathers and 42 690 (2.3%) of the mothers, with an overlap of 14 744 parents. A total of 70 723 infants were excluded due to missing information on parental education. Immigrants and Norwegians with education from foreign countries dominate this group.


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Table 2 Number of neonatal deaths and neonatal mortality rates (per 1000 live births) by maternal age, parity, plurality, marital status, and maternal and paternal education (years completed in 1998). Norway, three birth cohorts: 1967–1979, 1980–1989, and 1990–1998

 

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Table 3 Number of postneonatal deaths and postneonatal mortality rates (per 1000 live births) by maternal age, parity, plurality, marital status, and maternal and paternal education (years completed in 1998). Norway, three birth cohorts: 1967–1979, 1980–1989, and 1990–1998

 

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Table 4 Crude and adjusted odds ratios (OR)a of neonatal mortality by maternal and paternal education (years completed in 1998). 95% CI for crude and adjusted OR. All live born infants in Norway 1967–1998

 

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Table 5 Crude and adjusted odds ratios (OR)a of postneonatal mortality by maternal and paternal education (years completed in 1998). 95% CI for crude and adjusted OR. All live born infants in Norway 1967–1998

 

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Table 6 Neonatal and postneonatal mortality rates (per 1000 live births) and odds ratios by parents' education. All live births in Norway 1967–1979, 1980–1989, 1990–1998

 

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Table 7 Different estimates of educational differences in neonatal and postneonatal mortality. All live births in Norway 1967–1979, 1980–1989, 1990–1998

 
Statistical method
The association between parents' education and the risk of neonatal and postneonatal death, was estimated as absolute risk differences and by logistic regression analysis. The highest level of education was used as reference group. Crude odds ratios (OR) were obtained by using one independent variable in the model. Adjusted OR were obtained by using two to five independent variables: maternal and paternal education, maternal age, parity, and plurality. Thus, the effect of maternal education is adjusted for paternal education, maternal age, parity, and plurality. 95% CI are given for crude and adjusted OR (Tables 4 and 5). Population attributable fraction (PAF) was estimated to assess the proportion of neonatal and postneonatal deaths that would have been prevented if the risks among children of mothers with low and medium level of education were reduced to the risk among children of mothers with the highest level of education.19 The relative index of inequality (RII) were calculated as the OR from a logistic regression with the covariate given as the proportion with educational level higher or equal to oneself.20 This measure can be interpreted as the relative risk between the bottom and top of the educational hierarchy. SPSS statistical software was used in the analyses.21


    Results
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 Abstract
 Methods
 Results
 Discussion
 References
 
Characteristics of the study population are described in Table 1. Teenage pregnancies have decreased markedly during the last decades, from 9.7% in the 1970s to 3.4% in the 1990s. The proportion of mothers that are >=35 years has increased from 7.3% to 11.3% in the same period, while there are only minor changes in the distribution of parity. The proportion of multiple births has increased from 1.9% in the 1970s to 2.9% in the 1990s. The proportion of unmarried mothers varies over the period, with an increase in the 1980s and a return to 8.4% in the 1990s. Mothers' educational levels have increased considerably. The proportion of mothers in the lowest education group is less than a third of what it was in the 1970s, while the group of mothers with >12 years of education has almost doubled.

The risk of neonatal death decreased for all variables and categories during the 30-year period (Table 2). The risk of neonatal death declined most for infants whose mothers had low education. The reduction in absolute risk was higher among infants whose mothers had low education (6.2/1000) than among those with medium education (4.9/1000) and high education (3.5/1000). The risk difference in neonatal mortality between infants whose mothers had high and low education was reduced from 3.5/1000 in the 1970s to 0.9/1000 in the 1990s. There was a similar pattern for paternal education with a risk reduction of 6.1/1000 for infants whose fathers had <10 years of education and a risk reduction of 3.8/1000 for those whose fathers had >12 years of education. Among infants with unreported fathers the neonatal mortality rate is high in all three cohorts, with a risk reduction of 2.6 from the 1970s to the 1990s.

The postneonatal mortality rates were highest in the 1980s. However, the risk of postneonatal death decreased for all variables and categories from the 1970s to the 1990s (Table 3). For postneonatal deaths the reduction in absolute risk was lower among infants whose mothers had low education (0.6/1000) than among those with medium education (1.4/1000) and high education (2.0/1000). The risk difference between infants whose mothers had high and low education was only 0.7/1000 in the 1970s and increased to 2.0/1000 in the 1990s. For paternal education, the reduction in absolute risk was also lowest among infants whose fathers had low education (1.2/1000) than among those with high education (2.1/1000). In the group with unreported fathers the postneonatal mortality rates were high in all three cohorts, with a risk reduction of only 1.4 from the 1970s to the 1990s.

Relative risks of neonatal death by mothers' and fathers' education, estimated as crude and adjusted OR reveals that there is a persistent inverse association between parents' educational levels and neonatal mortality rates (Table 4). Again, infants whose fathers are not reported have a significantly higher risk than all other groups. When adjusting for maternal age, parity, and plurality, nearly no differences in the OR were found. When paternal or maternal education was included in the model the OR decreased in all three cohorts and did not remain significant in the 1990s.

There is a persistent inverse association between parents' educational level and postneonatal mortality rates (Table 5). The relative risks reveal an increasing inverse association between parents' educational levels and postneonatal mortality. When maternal age, parity, plurality, and paternal or maternal education were included in the logistic model, the OR decreased in all three cohorts, but remained statistically significant in the 1980s and 1990s.

The combined influence of mothers' and fathers' education on the risk of neonatal and postneonatal death for each cohort is shown in Table 6. Infants whose mothers and fathers both have high education are the reference groups. Infants with unreported fathers have a higher risk of both neonatal and postneonatal death irrespective of maternal education. Infants whose parents have low education have the highest risk in each cohort. The relative risk of neonatal death in this group increased from OR = 1.64 (95% CI: 1.5, 1.8) in the 1970s to OR = 1.79 (95% CI: 1.4, 2.3) in the 1980s, and decreased to OR = 1.67 (95% CI: 1.2, 2.4) in the 1990s (Table 6). The relative risk of postneonatal death increased from the 1970s to the 1990s in this group (1970s OR = 1.29, 95% CI: 1.1, 1.5, 1980s OR = 1.76, 95% CI: 1.4, 2.3, 1990s OR = 5.06, 95% CI: 3.4, 7.6).

Table 7 summarizes the findings. The risk differences between low and high maternal education for neonatal and postneonatal mortality are presented in the first row. The second row shows the relative risk between mothers with low and high education. The third row shows the relative risk between parents with low and high education. Both absolute and relative measures of inequality reveal decreased educational differences in neonatal mortality and increased educational differences in postneonatal mortality over time. The PAF confirms this pattern (fourth row). The proportion of neonatal deaths that could be attributed to low and medium maternal education decreased from 22.3 in the 1970s to 8.4 in the 1990s. In contrast, the proportion of postneonatal deaths that could be attributed to low and medium maternal education increased from 9.7 in the 1970s to 39.5 in the 1990s. For neonatal mortality, the RII for maternal education was lowest in the 1990s with a value of 1.32 (95% CI: 1.09, 1.60). In the 1980s RII was highest with a value of 1.91 (95% CI: 1.62, 2.25) and in the 1970s it was 1.72 (95% CI: 1.56, 1.89). For postneonatal mortality the RII for maternal education was 1.31 (95% CI: 1.13, 1.53) for the 1970s, 2.10 (95% CI: 1.74, 2.53) for the 1980s, and 4.00 (95% CI: 2.98, 5.36) for the 1990s.


    Discussion
 Top
 Abstract
 Methods
 Results
 Discussion
 References
 
The risk of infant death decreased considerably in all socio-economic groups from 1967 to 1998. Nevertheless, the inverse association between parents' education and risk of infant death persists throughout the period of observation. The different inequality measures used in this study reveal the same pattern; a noticeable decrease in socioeconomic differences in neonatal mortality and an increased inverse association between SES and postneonatal mortality. Another important finding is that infants whose fathers were not reported in the Medical Birth Registry have a high mortality rate irrespective of maternal educational level, and the risk reduction from the 1970s to the 1990s is small for this group of children.

The size and completeness of the dataset is a major strength in this study. It includes both medical and social information about the total population of live-born infants. Infant mortality is considered to be 100% ascertained in the Medical Birth Registry.22 The information on educational level from Statistics Norway is also considered to have high ascertainment and high reliability.23 These variables have been recorded in a consistent way over time by the Medical Birth Registry and Statistics Norway. Thus, it is unlikely that the results can be explained by selection bias, or changes in coding or classification.

We excluded births without information on parents' education. Immigrant parents dominate this group. Inclusion of this relatively small group did not influence our results significantly. However, a study of infant death and social class in immigrant groups raised a series of issues that we will address in a separate analysis.

Due to the categories we have chosen for education, most parents' educational level will not change after the age of 20. We assigned parents' education according to the highest level of education completed by the end of 1998.24 Thus, educational level of the parents of infants born to parents <20 years in the latest years of the observation period may be underestimated. However, this is a minor proportion of all parents. Assessment of the influence of cohabitation versus marriage is difficult due to registry routines. There is low reliability in reported cohabitation among unmarried mothers, particularly in the 1970s and 1980s. In addition, these mothers had monetary incentives to be registered as unmarried rather than cohabiting at that time. Also, other social norms and changing marriage patterns, as well as improved social welfare programmes, have made the comparisons between children born in or out of wedlock less meaningful.25,26 In summary the main variables included in the present analysis are robust, reliable, and valid. There is no reason to believe that the results are influenced by selection or information bias.

Today, the fact that a child is born out of wedlock in Norway does not imply that the mother is alone or in a difficult social or economic situation. Thus, it may be more reasonable to believe that unmarried mothers who do not give any information about the father are the ‘real’ single mothers. Missing information about the father is not due to stillbirth or early infant death. Further studies are needed to identify the specific risks associated with being an infant with an unreported father.

Maternal age and parity have repeatedly been reported to be associated with infant mortality as well as socioeconomic differences. The mean maternal age has increased in the period under study, while there were minor changes in the parity distribution. It seems likely, therefore, that adjustments for these variables only slightly modified the effect of education as a risk factor for infant death (Tables 4 and 5). When spouse's education was included in the logistic model, it was found that the relative differences between low and high education decreased. The influence of maternal education substantially reduced the relative importance of paternal education and vice versa. This shows the importance of studying educational level as an ‘individual variable’ and a ‘family status variable’.

Education is an indicator of both social and environmental conditions, and cultural resources. It is correlated with income and occupation and often used as a proxy for SES.8,27–29 However, the importance of education relative to other socioeconomic indicators has changed over the last decades as the level of education has increased. Womens' occupational activity has increased from 45% in 1972 to 78% in 1998.30 Work-related psychological strain can increase the risk of adverse pregnancy outcome, but work-related strain seems to be smaller in countries with highly developed social support systems and few work-related hazards.31 Occupations where workload and health hazards have been especially high have declined in the period under study. However, variations in work-related factors might still be potential causes of the higher rate of infant mortality among the lowest educated.32 Many service jobs are unskilled, and the growing service industry is occupied primarily by women and immigrant workers.33 Studies show that unskilled workers of this type face a higher risk of ill health compared with most other occupational groups.11,34

Health and mortality differences according to education have been attributed to the direct effects of education. The acquisition of health-related knowledge, optimized use of health services, and willingness to invest in human capital differ according to level of education. In addition, education may act through the promotion of high self-esteem and self-efficacy.27–29 Education influences the ability to solve problems. Thus, women from disadvantaged groups are not only more exposed to difficult life events or difficult life conditions, but also have less social support to limit the impact of such stressors.35–37

An important limitation of the study is that it lacks information on critical confounders such as cigarette smoking. Differences in smoking are probably the most important factors causing socioeconomic differences in infant mortality in developed countries.38–40 Pregnant women in Norway have reduced their cigarette smoking over the period under study, and the prevalence of daily smokers has decreased among the highest educated women whereas it has been more or less constant among those with low education.41,42 However, we do not know how the increased inequality in smoking habits affected our findings.

Different ways of measuring inequality complement each other as indicators of inequality trends.23 The attributable fraction is influenced by the proportion with low and medium educational level, but does not take into account that the reference group, high educational level, has become more common. The RII is the only measure used that explicitly allows for the changes in the distribution of educational levels. Thus, we consider this measure to be essential for our conclusion.

The RII tries to measure the difference between subjects at the top and bottom of the educational hierarchy. The RII were consistent with the relative risks and indicate that the social differences in neonatal mortality were higher in the 1980s than in the 1970s and 1990s, and that social differences in postneonatal mortality have increased from 1970s to the 1990s. The RII however extrapolates from a logistic regression model whose validity cannot be tested. There is reason to believe that parents with low education who gave birth in the 1990s represent less resourceful individuals than parents with low education who gave birth in the 1970s.

The association between neonatal death and educational level has decreased, and was relatively weak in the 1990s (Tables 4 and 6). Thus, the assumption is close that socioeconomic factors only marginally influence neonatal mortality in Norway, while high quality obstetric health services seem to override social inequalities. This implies that public health interventions aimed at reducing social inequalities are less likely to reduce the mortality rate further. It is more likely that improvements in the quality of obstetric and neonatal care will lead to further reduction in these deaths. They can be linked to the quality of perinatal and neonatal care, and the availability of experienced staff around the time of birth.43 It is known that most neonatal deaths are attributable to prematurity, birth defects, and ‘other diseases’ of the newborn. In further studies we therefore need to include gestational age and causes of death. In addition, it is of interest to divide neonatal death into early and late death in this period to find out if the closer one gets to birth, the smaller the social differences in neonatal mortality rates.1

Postneonatal mortality is reported to be associated more closely with SES than neonatal mortality.1,13,17,43 We believe that socioeconomic differences in the home environment of infants, which are likely to be important in determining levels of postneonatal mortality, may have received less attention than inequalities in obstetric health services.

Socioeconomic differences in infant mortality still persist in all the Nordic countries.43 Interestingly the patterns seem to differ between countries. In Sweden there were no socioeconomic differences in infant mortality in 1976, while in 1986, infant mortality had decreased more in the privileged group, and a socioeconomic difference had emerged.15 Another Swedish study found that the lowest infant mortality was not always seen in the highest educational group. Actually the stillbirth rate was increased and the neonatal mortality significantly increased (OR = 1.6) among women with >15 years of education.16 A Danish study revealed that socioeconomic differences in infant mortality had increased from 1982 to 1992.17 The decrease in mortality was primarily found among children whose mothers had high education, while infant mortality increased among those whose mothers had low education. A recent Finnish study with data from the 1990s found that the proportion of newborns with adverse pregnancy outcome had increased in the population, especially in the highest socioeconomic group.18 These findings were partly explained by the increase in certain maternal risk factors among women with high education. Mean maternal age increased due to postponed childbearing, and the proportion of multiple births increased due to more intensive use of advanced infertility treatments. The Swedish, Danish, and Finnish studies are controlled for smoking. The present Norwegian data does not confirm the finding that mothers with the highest education also have an increased risk of experiencing infant death. Neither do we observe an increased risk over time for those with low education, which is the case in Denmark. There is a need for studies with the same definition of infant death and other variables for comparisons between the Nordic countries.

Social and economic equality in health is a highly esteemed value and a major aim for Norwegian health policy. Our results indicate that this has succeeded relatively well, but that important inequalities persist even at a very low level of risk. A higher percentage of the social budget than in most other European countries is devoted to support for families and children.10 Medical care is mostly inexpensive, and access to antenal care, obstetric and child health care is available to all and free of cost. In addition, the utilization of these services is very high in the Norwegian population. However, the specific cause of the persistent inequality, and particularly of the high risk among infants with unreported fathers, remains to be identified.

An interesting question is whether infant death is a good indicator of the general health conditions and the welfare of a society when the risk of infant death is low. We concluded that despite infant mortality rates that are very low by all historical and international comparisons, the socioeconomic inequality in risk is an indicator of important societal phenomena.


KEY MESSAGES

  • The risk of infant death has decreased considerably in all socioeconomic status groups in Norway in the period 1967–1998.
  • The inverse association between educational level and pregnancy outcome is most pronounced for postneonatal death, and has increased over time.
  • The inverse association between parents' education and risk of infant death persists throughout the whole period of observation.
  • Infants whose fathers were not reported in the Medical Birth Registry have a high mortality rate irrespective of maternal educational level.

 


    References
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 Abstract
 Methods
 Results
 Discussion
 References
 
1 Antonowsky A, Bernstein J. Social class and infant mortality. Soc Sci Med 1977;11:453–70.[ISI][Medline]

2 Stockwell EG, Swanson DA, Wicks JW. Temporal variations in the relationship between infant mortality and economic status. Soc Indicator Research 1998;20:217–27.

3 Norwegian Statistical Yearbook. Oslo-Kongsvinger: Statistics Norway, 2002.

4 Whitehead M. A European perspective on health inequalities. Milbank Q 1998;76:469–92.[ISI][Medline]

5 Mackenbach JP, Kunst AE. Measuring the magnitude of socio-economic inequalities in health: An overview of available measures illustrated with two examples from Europe. Soc Sci Med 1997; 44:757–71.[CrossRef][ISI][Medline]

6 Townsend P, Davidson N. Inequalities in Health: The Black Report. Harmondsworth: Penguin Books, 1982.

7 Dahl E, Kjaersgaard P. Trends in Socioeconomic Mortality Differentials in Postwar Norway—Evidence in Interpretations. Sociol Health Illness 1993;15:587–611.[CrossRef][ISI]

8 Davey Smith G, Hart C, Hole D et al. Education and occupational social class: which is the more important indicator of mortality risks? J Epidemiol Community Health 1998;52:153–60.[Abstract]

9 Mhen H, Stronks D, Mackenbach JP. A lifecourse perspective on socio-economic inequalities in health: the influence of childhood socio-economic conditions and selection processes. Sociol Health Illness 1998;20:193–216.

10 Nordic Statistical Yearbook. Copenhagen: Nordic Council of Ministers, 2002.

11 Dahl E, Elstad JI. Recent changes in social structure and health inequalities in Norway. Scand J Public Health 2001;29:7–17.

12 Stoltenberg C, Magnus P, Lie RT et al. The influence of consanguinity and maternal education on the risk of stillbirth and infant death among children of ethnic Norwegian and Pakistan origin in Norway 1967–93. Am J Epidemiol 1998;148:452–59.[Abstract]

13 Arntzen A, Magnus P, Bakketeig LS. Different effects of maternal and paternal education on early mortality in Norway. Paediat Perinat Epidemiol 1993;7:376–86.[Medline]

14 Arntzen A, Moum T, Magnus P et al. The association between maternal education and postneonatal mortality. Trends in Norway, 1968–1991. Int J Epidemiol 1996;25:578–84.[Abstract]

15 Ericson, A, Ericson M, Källen B et al. Secular trends in the effect of socio-economic factors on birth weight and infant survival in Sweden. Scand J Soc Med 1993;21:10–16.[ISI][Medline]

16 Haglund B, Cnattingius S, Nordstöm ML. Social differences in late fetal death and infant mortality in Sweden 1985–1986. Paediatr Perinat Epidemiol 1993;7:33–44.[Medline]

17 Olsen O, Madsen M. Effects of maternal education on infant mortality and stillbirths in Denmark. Scand J Public Health 1999;27:128–36.[CrossRef][ISI][Medline]

18 Gissler M, Meriläinen J, Vuori E, Hemminki E. Register-based monitoring shows decreasing socioeconomic differences in Finish perinatal health. J Epidemiol Community Health (in press), 2002.

19 Rothmann KJ, Greenland S. Modern Epidemiology. Philadelphia PA: Lippincott-Raven Publishers, 1998, pp. 295–97.

20 Mackenbach JP, Kunst AE. Measuring the magnitude of socio-economic inequalities in health: An overview of available measures illustrated with two examples from Europe. Soc Sci Med 1997; 44:757–71.[CrossRef][ISI][Medline]

21 SPSS, Inc. SPSS base system reference guide. Release 6.1. Chicago, SPSS, Inc, 1994.

22 Births in Norway through 30 years (1967–1996). Bergen: Medical Birth Registry of Norway, 1997.

23 Dahlheim E. More education in the luggage. Samfunnspeilet 2001;2:48–54. Oslo-Kongsvinger: Statistics Norway, 2001.

24 Kunst AE, Bos V, Mackenback JP. Guidelines on Monitoring Socio-Economic Inequalities in Health in European Countries. Rotterdam: Erasmus University, 2001.

25 Arntzen A, Moum T, Magnus P et al. Marital status as a risk factor for fetal and infant mortality. Scand J Soc Med 1996;24:36–42.[ISI][Medline]

26 Bennett T, Braveman P, Egerter S et al. Maternal marital status as a risk factor for infant mortality. Fam Plann Perspect 1994;26:252–56.[ISI][Medline]

27 Fuchs VR. Economics, health and post-industrial society. Milbank Mem Fund Q 1997;57:153–82.

28 Pincus T, Challahan LF. Associations of low educational level and poor health status: behavioural, in addition to demographic and medical, explanations? J Clin Epidemiol 1994;47:355–61.[ISI][Medline]

29 Blane D, Bartley M, Davey Smith G. Disease etiology and materialist explanations of socioeconomic mortality differentials. Eur J Public Health 1997;7:385–91.[Abstract]

30 NOS Labour Marked Statistics. Oslo-Kongsvinger: Norwegian Statistics, 2000.

31 Henriksen TB, Hedegaard M, Secher NJ. The relation between psychological job-strain, and pre-term delivery and low birth-weight for gestational age. Int J Epidemiol 1994;23:764–74.[Abstract]

32 Kramer MS, Goulet L, Lydon J et al. Socio-economic disparities in preterm birth: causal pathways and mechanisms. Peadiatr Perinat Epidemiol 2001;15:104–23.[CrossRef]

33 Esping-Andersen G. Changing classes. Stratification and Mobility in Post-Industrial Societies. London: Sage, 1993.

34 Fox J. Health Inequalities in European Countries. Aldershot: Gover, 1989.

35 Seguin L, Potvin L, Loiselle J et al. Chronic stressors, social support, and depression during pregnancy. Obstet Gynecol 1995;85:583–89.[Abstract/Free Full Text]

36 Cohen S. Psychosocial models of the role of social support in the etiology of physical disease. Health Psychol 1998;7:269–97.

37 Hoffman S, Hatch MC. Stress, social support and pregnancy outcome: a reassessment based on recent research. Paediatr Perinat Epidemiol 1996;10:380–405.[ISI][Medline]

38 Nordentoft M, Lou HC, Hansen D et al. Intrauterine growth retardation and premature delivery: the influence of maternal smoking and psychosocial factors. Am J Public Health 1996;86:347–54.[Abstract]

39 Woolbright LA. The effect of maternal smoking on infant health. Popul Res Policy Rev 1994;13:327–39.[ISI]

40 Wisborg K, Kesmodel U, Henriksen TB et al. Exposure to tobacco smoke in utero and the risk of stillbirth and death in the first year of life. Am J Epidemiol 2001;154:322–27.[Abstract/Free Full Text]

41 Eriksson KM, Haug K, Salvesen KÅ et al. Smoking habits among pregnant women in Norway 1994–1995. Acta Obstet Gynecol Scand 1998;77:159–64.[CrossRef][ISI][Medline]

42 National Strategy Plan for Preventing Cigarette Smoking, 1999–2003. Oslo: Ministry of Health and Social Affairs, 1999.

43 Cnattingius S, Haglund B. Socio-economic factors and feto-infant mortality. Scand J Soc Med 1992;20:11–13.[ISI][Medline]