The effect of smoking and hypertensive disorders on abruptio placentae in Norway 1999-2002

Objective. To assess the effects of smoking and hypertension on abruptio placentae and whether the effect of hypertension is modified by smoking and vice versa. To quantify the proportion of abruptio placentae cases attributable to hypertension and smoking. Design. Registry based national cohort study. Materials and methods. 814 cases of abruptio placentae among a total of 211,868 births in Norway during the period 1999 - 2002. Logistic regression analysis was used to assess whether the risk of abruptio placentae is influenced by maternal smoking habits and hypertensive disorders and to evaluate interactions in the effects on abruptio placentae of smoking and hypertension. Results. In occasionally and daily smokers at the onset of pregnancy, abruptio placentae occurred in 5.4 and 5.7 per 1000, respectively, against 3.4 per 1000 in non-smokers ( adjusted odds ratios (ORs) for maternal age and birth order 1.6 - 1.7). No significant dose-response effect was found. Adjusted ORs for abruptio placentae in women with pre-eclampsia superimposed on chronic hypertension, transient hypertension, mild and severe pre-eclampsia, and pre-eclampsia with onset before 34(th) weeks of gestation were significantly increased ( ORs 1.6 - 4.1). The combined effects of smoking and hypertensive disorder were additive. Hypertension explained 3.3% of abruptio placentae, while smoking explained 6.9% of abruptio placentae cases. Conclusions. The effects on abruptio placentae of hypertensive pregnancy disorders and smoking are independent and additive, indicating that smoking and hypertension have their effects though separate mechanisms. If smoking were eliminated in pregnant women, the number of abruptio placentae cases would be reduced by 7%.