Relationship between pregnancy-induced hypertension and placenta previa: a population-based study.

Objective: Our purpose was to investigate, in a large population-based cohort, the hypothesis that the risk of pregnancy-induced hypertension is lower among pregnancies complicated by placenta previa compared with pregnancies occurring in women with fundally implanted placentas.

Study Design: Data for this retrospective cohort study were derived from the computerized Atlee perinatal database of the Reproductive Care Program, Nova Scotia, Canada. Women who were delivered in the province between 1980 and 1993 were included in the study. Patients with pregnancy-induced hypertension were clinically diagnosed by the presence of elevated blood pressure, proteinuria, or edema. The risk of pregnancy-induced hypertension was compared between women diagnosed with placenta previa and those with a normally implanted placenta, after adjustment for potential confounders through multivariable logistic regression models based on the method of generalized estimating equations.

Results: During the 14 years (1980 to 1993), 121,082 singleton pregnancies were registered in the program, 416 (0.4%) of which had a confirmed diagnosis of placenta previa. Women with chronic hypertension had a relative risk of 1.2 (95% confidence interval 0.4 to 3.7) for placenta previa compared with normotensive women. However, the risk of pregnancy-induced hypertension was reduced by half among those with placenta previa (relative risk 0.5, 95% confidence interval 0.3 to 0.7). Adjustments for potential confounders, including maternal age, parity, prepregnancy body weight, prior cesarean delivery, prior spontaneous or induced abortions, and cigarette smoking, had no influence on this association. Analyses on the basis of stratification of women by parity (nulliparous vs multiparous), cigarette smoking (smoker vs nonsmoker), and gestational duration (< 28, 28 to 32, 33 to 36, and > 37 completed weeks) consistently showed reduced risks for pregnancy-induced hypertension among women with placenta previa, indicating that the association was not a result of shortened duration of gestation among women with placenta previa.

Conclusions: The results from this study clearly show a decreased frequency of pregnancy-induced hypertension among those pregnancies with placenta previa. We speculate that the pathophysiologic mechanisms for this finding may be due to altered placental perfusion seen among women diagnosed with placenta previa.