Sex ratio at birth following prenatal maternal exposure to severe life events: a population-based cohort study.

BACKGROUND A reduction in the sex ratio at birth has been linked to maternal condition during and before pregnancy. A recent study reported an association between maternal exposure to severe life events and sex ratio at birth using the Danish national register. We attempted to replicate that study using a new Danish cohort. METHODS Mothers of all singleton live births (n = 1.35 million births) in Denmark, between 1 January 1980 and 31 December 2002, were linked to data on their children and partners. The old cohort consisted of babies born between 1980 and 1992 (n = 699 362), whereas the new cohort consisted of babies born between 1993 and 2002 (n = 633 451). We defined exposure as death or serious illness in older children and partners in the first trimester or in the 6 months before conception. Sex ratio at birth was defined as the proportion of male live births. RESULTS During the study period, there were 1,349,099 singleton live births (692,870 boys and 656,229 girls). The sex ratio at birth in the new cohort was 0.5134. In the new cohort, prenatal exposure to severe life events was not associated with a reduction in the sex ratio at birth [relative risk = 1.00 (95% confidence interval: 0.95-1.05)]. CONCLUSIONS In the new cohort, we did not find strong evidence that, in a stable western population, prenatal exposure to severe life events is associated with a reduction in the sex ratio at birth.