Relationship between placenta previa and preeclampsia: A population-based study

OBJECTIVE: To investigate in a large population-based cohort, the relationships between preexisting hypertension and placenta previa, as well as the effect of placenta previa on preeclampsia. STUDY DESIGN: Data for this retrospective cohort study were derived from the computerized Atlee perinatal data base of the Reproductive Care Program of Nova Scotia, Canada. Women who delivered in the province between 1980 and 1993 were included in the study. Patients with preeclampsia were clinically diagnosed by the presence of elevated blood pressure, proteinuria, or edema. The risk of preeclampsia was compared between women with placenta previa and those having normally-implanted placentae, after adjustment for potential confounders through multivariable logistic regression through the method of generalized estimating equations. RESULTS: During the 14 years (1980 to 1993), 121,082 singleton pregnancies were registered in the program, of whom 416 (0.4%) had a confirmed diagnosis of placenta previa. Women with chronic hypertension had a relative risk (RR) of 1.2 for placenta previa (95% confidence interval (CI) 0.4-3.7) when compared to normotensive women. However, the risk of preeclampsia was reduced by half among those with placenta previa (RR = 0.5, 5% CI 0.3-0.7) when compared to normotensive women. Adjustments for confounders including maternal age, parity, pre-pregnancy body weight, prior cesarean delivery, prior spontaneous and induced abortions, and cigarette smoking had no influence on this association. Stratification of women based on their parity (nulliparous versus multiparous), cigarette smoking (smoker versus non-smoker), and gestational age ( <37 versus over 37 weeks) revealed reduced risk for preeclampsia. CONCLUSIONS: The results from this study clearly show a decreased incidence of preeclampsia 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.