INTRODUCTION: Maternal smoking during pregnancy has been associated with an increased risk of stillbirth. Only a few studies have been conducted to determine whether smoking affects the risk of antepartum and intrapartum stillbirth differently or whether smoking cessation in early pregnancy reduces the risk. Previous results are inconclusive. We addressed these questions in a large Danish population-based cohort study.

MATERIAL AND METHODS: From the Danish Medical Birth Register, we identified 841 228 singleton births in Denmark between 1997 and 2010 and gathered detailed information on maternal smoking during pregnancy and the vital status of the infant. Associations (odds ratios with 95% confidence intervals) between maternal smoking and risk of stillbirth overall and separately for antepartum and intrapartum stillbirth were analyzed using logistic regression models (generalized estimating equations), adjusting for potential confounders.

RESULTS: Any smoking during pregnancy increased the risk of stillbirth, both overall (odds ratio 1.42, 95% confidence interval 1.30-1.55) and for antepartum (odds ratio 1.38, 95% confidence interval 1.25-1.53) and intrapartum (odds ratio 1.52, 95% confidence interval 1.18-1.96) stillbirths. Women who quit smoking at the beginning of the second trimester at the latest had no increased risk of stillbirth overall (odds ratio 1.03, 95% confidence interval 0.80-1.32).

CONCLUSIONS: Maternal smoking during pregnancy increases the risk of stillbirth, both overall and for antepartum and intrapartum stillbirth separately. Women who quit smoking in the beginning of their pregnancy reduce their risk compared with that of non-smokers.