Introduction A large body of literature has reported associations between socioeconomic position (SEP) and adverse pregnancy outcomes even in affluent egalitarian welfare states. It is unclear if this is interpretable as a causal effect of SEP. This study seeks to explore the nature of this relationship by examining women who change SEP between pregnancies and women who are siblings but are different in terms of SEP.
Methods Data consist of 471 215 live born singletons born in Denmark 1997–2007, who has at least one sibling or one first cousin. We examined maternal educational attainment and household income in relation to preterm birth and small for gestational age using Cox regression.
Results Household income was not related to the outcomes in cohort analyses, within mothers who were siblings or children who were siblings. Maternal education was associated with preterm birth only in the cohort analyses, where the least educated women had the highest risk. This suggests that factors that originate in the mother's early life, for example, shared genes or early life environment, explain the association. Maternal education was inversely associated with small for gestational age in cohort analyses and within mothers who were siblings, but not between children who are siblings. This suggests that the association was explained by factors that were persistent over the mother's pregnancies, but not shared between the mother's siblings.
Conclusion The association between maternal education and the outcomes cannot be interpreted as a causal effect. The association is likely caused by factors established in childhood or young adulthood.
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