Maternal smoking in pregnancy and risk for congenital malformations: results of a Danish register-based cohort study

Objective. To examine the association between maternal smoking during pregnancy and risk for congenital malformations. Design. Population-based prospective cohort study. Setting. Denmark. Population. A total of 838 265 singleton liveborn babies delivered in Denmark between 1997 and 2010 and registered in the Danish Medical Birth Register containing detailed information on smoking during pregnancy and congenital malformations. Methods. Associations [odds ratios (OR) with 95% CI] between maternal smoking and risk for various groups of congenital malformations, investigated using the generalized estimating equation for binary outcomes, with adjustment for potential confounders. Main outcome measures. Groups of congenital malformations. Results. Ever smoking during pregnancy did not increase the overall risk for congenital malformations, but increased risks were observed for multiple malformations (i.e. when two or more malformations are diagnosed in a child) (odds ratio 1.06, 95% confidence interval 1.01-1.10) and various main groups of congenital malformations including the cardiovascular system (odds ratio 1.13, 95% confidence interval 1.07-1.19), the respiratory system (odds ratio 1.25, 95% confidence interval 1.11-1.41), the digestive system (odds ratio 1.15, 95% confidence interval 1.07-1.24) and oral clefts (odds ratio 1.29, 95% confidence interval 1.14-1.46), as well as for some specific congenital malformations including cardiac septal defects, malformations of the pulmonary and tricuspid valves, malformations of the great arteries, pyloric stenosis and clubfoot. Infants of women who quit smoking during the first two trimesters had no increased risk for most groups of congenital malformations. Conclusions. Maternal smoking increases the risk for a number of congenital malformations. Future smoking cessation programs should focus on this adverse health aspect in order to encourage more women to quit smoking before or in early pregnancy.