1 National Institute of Occupational Health, Oslo, Norway
2 Division of Military Medical Research and Development, Joint Norwegian Medical Services, 0753 Oslo, Norway
3 Medical Birth Registry of Norway, Locus of Registry Based Epidemiology, University of Bergen, 5018 Bergen, and Norwegian Institute of Public Health, 0403 Oslo, Norway
Correspondence: Petter Kristensen, National Institute of Occupational Health, PO Box 8149 Dep, 0033 Oslo, Norway. E-mail: petter.kristensen{at}stami.no
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Abstract |
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Methods Through linkage between several national registers containing personal information from birth into adult age we established a longitudinal, population-based cohort study. Study participants were all 308 829 singletons born in Norway in 19671971 as registered by the Medical Birth Registry of Norway who were national residents at age 29. The study outcome was unemployment defined as a lack of personal income among people who were not under education in the calendar year of their 29th birthday as registered by the National Insurance Administration and Statistics Norway.
Results Birthweight below the standardized mean was associated with unemployment. The risk of unemployment increased by decreasing birthweight for both women and men and also after adjustment for potential confounding factors. The association was evident both in people with or without social disadvantage, as well as people with or without childhood disease. Still, birthweight below the standardized mean explained much less of the unemployment risk than did social disadvantage (attributable fractions 8.0% versus 28.3% for women and 10.0% versus 40.2% for men).
Conclusion Birthweight below the standardized mean was independently associated with unemployment at age 29, also in the normal birthweight range.
Accepted 6 January 2004
Low birthweight is associated with a number of adverse health outcomes in adult age, including coronary heart disease,1 limiting illness,2 disability, and mortality,3 as well as psychological distress.4 Associations are also observed with cognitive function,5,6 educational achievement,57 and other functional outcomes in adult age,7,8 while the association with employment is more indefinite.7 The association between birthweight and adult function seems to be independent of social class of origin with no heterogeneity on a multiplicative scale.6,7 Being associated with both long-term health and socioeconomic status, low birthweight is likely to be involved in selection processes that add to social inequalities in health. We still know little about the strength of such social selection but it seems to be limited.9,10
To study the association between birthweight and work participation in young adult age, we examined data from several population-based national registers in Norway. The objective was to quantify the association between birthweight and subsequent work participation. We also had an aim to study how birthweight, early social disadvantage, and childhood disease interact in determining work participation. In principle, social disadvantage and childhood disease might confound the association between birthweight and work participation; they might either modify the effect of birthweight or birthweight and other factors might be associated with work participation independently.
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Methods |
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Main determinants and outcome measures under study
Linkage provided longitudinal data for the index person and their parents. Data were updated at least through the year 2000 on social benefits, pensionable income, education, death, and emigration, as well as marital status and childbirth.
The main study determinant was birthweight registered in birth records. Mean birthweights were 3443 g (SD 512 g) for girls and 3571 g (SD 541 g) for boys. We standardized birthweight for gender in order to compare birthweight categories between genders. This procedure gave a birthweight SD score or z score where one unit increase in z score equals an increase in birthweight of 1 SD; the standardized mean birthweight has a zero score. The z scores were divided into nine 1 SD categories. When appropriate, z scores were collapsed into five or two categories.
The study outcome was lack of income among people who were in education in the calendar year of their 29th birthday. Personal earnings are reported annually to the National Insurance Administration to estimate forthcoming old age pension. This pensionable income is recorded by the Administration in units that are adjusted regularly in accordance with changes in the general income level. An income of half a unit (equalling GBP 2500 in 2002) is the limit entitling sickness absence compensation; accordingly, we defined an income lower than half a unit as lack of income. Annually in October, ongoing education is recorded in the education register. We considered people who were in education in October in the calendar year of their 29th birthday, or in October the preceding year, to be in education. We termed the dichotomous outcome (lack of income and no ongoing education) as unemployment.
We considered other variables potentially related to unemployment. The Medical Birth Registry provided data on gestational age, birth order, demographic data for parents, maternal health, and perinatal health including birth injuries and congenital malformations.12 From the Central Population Register we obtained data on parental marital status between 1985 and 2002, both parents' number of children, and date of death or emigration of the index person as well as both parents. We received data on annual pensionable income and pension benefits for both parents in the income and benefit registers, respectively. Annual recordings of benefits13 for the index person during childhood were retrieved in the Benefit Register. The Education Register provided the highest educational achievement for the index person as well as both parents (parents, in the year of the index person's 16th birthday).
From the available parental data we constructed an indicator of social disadvantage: father's identity not stated in the birth record; mother not married at index person's birth or in 1985; maternal chronic disease before pregnancy stated in birth record; mother and father having unequal number of children according to the Central Population Register; parental death (before age 25 of the index person); mean maternal income less than half the basic amount during adolescence (1724 years) of the index person; mean paternal income less than half the basic amount during all index person's age periods (06, 716, 1724 years); lack of registered education for mother or father. Data in the Medical Birth Registry and Benefit Register allowed us to create a childhood disease variable: insurance benefit due to chronic disease before age 7 years; intracranial injury/bleeding at birth (International Classification of Diseases, Eighth Revision [ICD-8] 431); birth injury (ICD-8 772), congenital malformation (ICD-8 740759, or additional diagnoses as specified in ref. 12). Population distributions and unemployment frequencies for social disadvantage, childhood disease, and categories of gestational age, birth order, and educational attainment are provided in Table 1.
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Analysis |
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Social disadvantage, childhood disease, gestational age, and birth order were all potential confounders and were included in the multivariate models. First we estimated crude and adjusted OR for birthweight. OR of the principal covariates, social disadvantage, and childhood disease, were also computed.
Educational attainment was strongly correlated with unemployment (Table 1). We considered level of education as a potential intermediate between birthweight and unemployment. To achieve further insight into the relations of these factors we examined birthweight effects on unemployment within strata of educational attainment by using the likelihood ratio test (change in deviance by introducing birthweight into the model).
Interaction effects (heterogeneity on a multiplicative scale) between birthweight and social disadvantage, respectively birthweight and childhood disease, were assessed by likelihood ratio tests. We computed the likelihood ratio statistic (change in deviance with corresponding degrees of freedom and P-value) by adding the product term to a model that included the individual variables. We considered interaction to be present if the product term improved the model significantly (P < 0.05).
Furthermore, we estimated the joint effects of birthweight, social disadvantage, and childhood disease in a 2 x 2 x 2 contingency table. We computed population attributable fractions of birthweight, social disadvantage, and childhood disease. Population attributable fraction was defined as the fraction of observed cases, with or without the factor(s), which would not have occurred if the factor(s) had not had an effect.15
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Results |
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Negative z score categories of birthweight were increasingly associated with unemployment among people both with and without social disadvantage and for both genders (Table 4). The likelihood ratio test demonstrated no interaction: the likelihood ratio statistics (4 d.f.) were 1.675 (P = 0.7953) and 4.434 (P = 0.3505) for women and men, respectively.
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Discussion |
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Our study adds to the literature showing associations between birthweight and several outcomes related to cognitive abilities, health, and social function in adult age.28,16 An effect across the normal range of birthweight below the mean is also in agreement with studies on other outcomes in adulthood.26,8,16 It is also interesting that the effects of birthweight and social disadvantage in our study were similar to what others have found: both variables act in an independent fashion on a multiplicative scale.57 Furthermore, early social disadvantage seems to have larger impact than birthweight in determining adult functional attainment.6,7
Strauss reported that small for gestational age was not associated with unemployment and weekly working hours at age 26 in the British 1970 birth cohort, whereas a negative association with professional and managerial jobs and weekly income was found.7 We should keep in mind that Strauss used a different measure of size at birth, but this apparent discrepancy with our results might be due to low power in Strauss' study.7 Strauss found a 0.6% point increase in unemployment in association with small for gestational age, which was disregarded because it was not statistically significant.7 The upper limit of the 95% CI of his estimate was a 2.4% increase, which is a more than 50% unemployment increase compared with the reference group (ref. 7, Table 3). Birthweight was inversely associated with unemployment at age 23 in the 1958 British birth cohort,16 but the separate effect on unemployment is difficult to interpret because it was combined with other outcome measures. Direct comparisons between Britain and Norway should be made with caution. The fate of vulnerable groups in the labour market depends not only on macroeconomic development but also policy measures, which may vary between countries.17
The main strengths of our study are the complete follow up of subjects, the large size, and the availability of data from several national registers throughout the life course. The overall loss in follow up was only 5% and due to death and emigration. In this respect, our historical register follow-up study has an advantage over conventional prospective life course cohorts, e.g. the British birth cohorts27 with a loss at adult age of approximately half the participants. The large size allows subset analyses and, at the same time, yields rather robust effect estimates in extreme birthweight categories. The unique national identification number assigned to all residents in Norway allows data linkage between several registers. With a broad range of social and health-related data from birth to adult age we had the opportunity to examine combined effects of birthweight, family background, and disease.
One important limitation of our study is related to some of the data at hand. Some registers are mainly administrative and data often tend to be blunt and proxies for what we are really looking for in research. However, we believe that the blunt data problem would not affect birthweight, which was the main study determinant. We could have a larger problem in making sound inferences because we have to rely on a variable likely to be a proxy for unemployment. Unemployment according to our definition is likely to be heterogeneous and lack precision. This could result in biased results if the degree of error were dependent on the determinants under study, which might be the case in some instances. Our proxy data problem can be contrasted to the more tailored data in the prospective British birth cohorts, e.g. repeated measurements of cognitive and intellectual capacity from childhood into adult age.57 Such data could prove important in explaining the relation between birthweight and work participation.
We found that the attributable fraction of unemployment that could be accounted for by birthweight was less than 10%, which was considerably smaller than that of social disadvantage. Dividing men into two equal halves based on birthweight showed that men with a birthweight below the mean had a 22% odds increase of being unemployed whereas the division of men into two approximate equal parts based on social factors yielded an odds increase of 137% among the disadvantaged. These numbers should not be automatically interpreted as a preventive potential. Work participation is mainly influenced by macro level societal conditions, and not by individual factors.18,19 We could however argue that a reduction of the impact of birthweight or social disadvantage on work participation would result in more equity in the labour market, and this could be important in its own right. To influence the possible effects of individual factors we would need an understanding of the mechanisms behind the relation between birthweight and adult social functioning. These mechanisms are as yet unknown. Thus, an effect of genetic factors on birthweight as well as adult social functional abilities cannot be ruled out. Still, the reported association between decreasing birthweight and a monotonous decrease in cognitive function in childhood and young adult age5,6 could prove important in understanding the birthweight relation with work participation. The present data provide only limited opportunities for answering these questions, but show that the birthweight effect was largely restricted to subjects with limited educational attainment. This might in turn be interpreted as support for the hypothesis that the birthweight effect is related to central nervous system development and early cognitive function. However, it is not clear whether birthweight in the normal range plays a role as determinant in a causal pathway or is merely correlated with the causal factor.
KEY MESSAGES
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Acknowledgments |
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References |
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