Smoking during pregnancy increases risks of various obstetric complications: A case-cohort study of the Japan Perinatal Registry Network database

Background: The adverse effects of maternal smoking on the health of pregnant women have been examined mostly on a disease-by-disease basis. The aims of this study were to evaluate simultaneously the effects of smoking during pregnancy on various obstetric complications, using data from a large medical database, and to investigate the expediency of using a case-cohort design for such an analysis. Methods: A case-cohort study was conducted within the Japan Perinatal Registry Network database. Perinatal information on infant deliveries was entered into the database at 125 medical centers in Japan. The base population of the study was 180 855 pregnant women registered in the database from 2001 through 2005. The outcome measures were the incidences of 11 different obstetric complications. Logistic regression models were used to estimate ageadjusted risk ratios (aRRs) and relative excess incidence proportions (REIs). Results: The overall prevalence of smoking during pregnancy was 5.8% in the base cohort, and the prevalence was higher among younger women. A comparison of the cases and control cohort showed that smokers during pregnancy had statistically significant higher risks for preterm rupture of the membrane (aRR: 1.67, 95% confidence interval [CI]: 1.43-1.96; REI: 40.2%, 95% CI: 29.9%-49.1%), chorioamnionitis (1.65, 1.36-2.00; 39.4%, 26.4%-50.0%), incompetent cervix (1.63, 1.35-1.96; 38.5%, 25.8%-49.1%), threatened premature delivery (1.38, 1.17-1.64; 27.7%, 14.5%-38.9%), placental abruption (1.37, 1.10-1.72; 27.1%, 8.8%-41.7%), and pregnancy-induced hypertension (1.20, 1.01-1.41; 16.4%, 1.2%-29.3%). Conclusions: Maternal smoking was associated with a number of obstetric complications. This highlights the importance of smoking cessation during pregnancy. In addition, case-cohort analysis proved useful in estimating RRs for multiple outcomes in a large database.