Maternal smoking in pregnancy and sex differences in perinatal death between boys and girls

The sex difference in perinatal mortality in developed countries is largely unexplained. The current study evaluated the differences in the impact of maternal smoking during pregnancy on the risk of perinatal death between males and females. The analysis involved 11,469 and 9,404 newborns derived from two population-based birth cohorts in Northern Finland, for 1966 and 1985-86, respectively. The perinatal mortality rate was 23 per thousand in the 1966 cohort and 9 per thousand in the 1985-86 cohort. The rate ratio (RR) for mortality for males over females is 1.15 and 1.60 in the two cohorts, respectively. Among children whose mothers smoked during pregnancy, the RR was 2.2 (95% CI 1.0, 4.7) for the former cohort and 4.8 (95% CI 1.5, 15.2) for the later cohort; and among the children whose mothers did not smoke the corresponding RR was 1.2 (95% CI 0.9, 1.6) and 1.1 (95% CI 0.6, 1.9). Maternal smoking during pregnancy could be an important determinant accounting for the excess perinatal death for males over females. Our results encourage evaluation of the findings among other populations.