Maternal smoking in pregnancy and birth defects: a systematic review based on 173 687 malformed cases and 11.7 million controls

BACKGROUND ; There is uncertainty over whether maternal smoking is associated with birth defects. We conducted the first ever comprehensive systematic review to establish which specific malformations are associated with smoking. METHODS ; Observational studies published 1959-2010 were identified (Medline), and included if they reported the odds ratio (OR) for having a non-chromosomal birth defect among women who smoked during pregnancy compared with non-smokers. ORs adjusted for potential confounders were extracted (e.g. maternal age and alcohol), otherwise unadjusted estimates were used. One hundred and seventy-two articles were used in the meta-analyses: a total of 173 687 malformed cases and 11 674 332 unaffected controls. RESULTS ; Significant positive associations with maternal smoking were found for: cardiovascular/heart defects [OR 1.09, 95% confidence interval (CI) 1.02-1.17]; musculoskeletal defects (OR 1.16, 95% CI 1.05-1.27); limb reduction defects (OR 1.26, 95% CI 1.15-1.39); missing/extra digits (OR 1.18, 95% CI 0.99-1.41); clubfoot (OR 1.28, 95% CI 1.10-1.47); craniosynostosis (OR 1.33, 95% CI 1.03-1.73); facial defects (OR 1.19, 95% CI 1.06-1.35); eye defects (OR 1.25, 95% CI 1.11-1.40); orofacial clefts (OR 1.28, 95% CI 1.20-1.36); gastrointestinal defects (OR 1.27, 95% CI 1.18-1.36); gastroschisis (OR 1.50, 95% CI 1.28-1.76); anal atresia (OR 1.20, 95% CI 1.06-1.36); hernia (OR 1.40, 95% CI 1.23-1.59); and undescended testes (OR 1.13, 95% CI 1.02-1.25). There was a reduced risk for hypospadias (OR 0.90, 95% CI 0.85-0.95) and skin defects (OR 0.82, 0.75-0.89). For all defects combined the OR was 1.01 (0.96-1.07), due to including defects with a reduced risk and those with no association (including chromosomal defects). CONCLUSIONS ; Birth defects that are positively associated with maternal smoking should now be included in public health educational materials to encourage more women to quit before or during pregnancy.