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 larg e prospective cohort study that collected detailed information on blood pre ssure, proteinuria, smoking, and placental morphologic and histologic chara cteristics. A total of 9651 healthy primigravid women without chronic hyper tension 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 prena tal visits were included. Gestational hypertension was defined as diastolic blood pressure greater than or equal to 90 mm Hg on 2 occasions from 24 we eks' gestation to 2 weeks post partum. Preeclampsia was defined as gestatio nal hypertension plus greater than or equal to 2 urine samples containing g reater 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 asso ciated ina dose-response pattern with reduced risks of gestational hyperten sion and preeclampsia. For women who smoked greater than or equal to 10 cig arettes/d the relative risks with respect to women who had never smoked wer e 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 eff ect was observed both for mild and severe gestational hypertension and for preeclampsia. The more and the longer a woman had smoked previously the low er was her risk of development of hypertension during pregnancy. This assoc iation could not be explained by confounding factors, by changes in placent al morphologic or histopathologic characteristics, by maternal net weight g ain, or by elevated liver enzyme bioactivity. CONCLUSION: Smoking is associated with a reduced risk of hypertension durin g pregnancy. The protective effect appears to continue even after cessation of smoking. Further basic research-on this issue is warranted.