The puzzling association between smoking and hypertension during pregnancy

OBJECTIVE: The object of this study was to examine the association between maternal smoking and hypertension during pregnancy. STUDY DESIGN: We used data from the Collaborative Perinatal Project, a large prospective cohort study that collected detailed information on blood pressure, proteinuria, smoking, and placental morphologic and histologic characteristics. A total of 9651 healthy primigravid women without chronic hypertension who had been enrolled in the study at the first or second trimester (average 18 weeks' gestation) and had had greater than or equal to 3 prenatal visits were included. Gestational hypertension was defined as diastolic blood pressure greater than or equal to 90 mm Hg on 2 occasions from 24 weeks' gestation to 2 weeks post partum. Preeclampsia was defined as gestational hypertension plus greater than or equal to 2 urine samples containing greater than or equal to 1+ protein according to dipstick measurement during the same gestational period. RESULTS: After we controlled for prepregnancy body mass, age, socioeconomic status, and race, both past smoking and smoking during pregnancy were associated ina dose-response pattern with reduced risks of gestational hypertension and preeclampsia. For women who smoked greater than or equal to 10 cigarettes/d the relative risks with respect to women who had never smoked were 0.6 (95% confidence interval, 0.4-0.9) for gestational hypertension and 0.5 (95% confidence interval, 0.4-0.7) for preeclampsia. This protective effect was observed both for mild and severe gestational hypertension and for preeclampsia. The more and the longer a woman had smoked previously the lower was her risk of development of hypertension during pregnancy. This association could not be explained by confounding factors, by changes in placental morphologic or histopathologic characteristics, by maternal net weight gain, or by elevated liver enzyme bioactivity. CONCLUSION: Smoking is associated with a reduced risk of hypertension during pregnancy. The protective effect appears to continue even after cessation of smoking. Further basic research-on this issue is warranted.