Reduced frequency of hypertensive disorders in placenta previa.

The isthmic segment of the uterine artery's ascending branch has a freer course and wider diameter than distal parts of the vessel. Therefore, we assumed that this arterial segment would provide better blood flow and prevent hypoxia of the trophoblast. As a result, placenta previa pregnancies would be complicated by hypertensive disorders less often than are pregnancies with normally implanted placentas. To test this hypothesis, 491 placenta previa pregnancies, among a population of 106,866 pregnant women, were compared with pregnancies with normally implanted placentas. Clinically meaningful and statistically significant reductions in the rates and risks of hypertensive disorders were found in placenta previa pregnancies (P = .002, relative risk = 0.44, 95% confidence interval 0.25-0.78). The differences persisted when primiparous and multiparous women were examined separately and when preterm and term deliveries were separated. In a multivariate logistic regression analysis, patients with placenta previa had a third of the risk for hypertensive disorders compared with pregnant women with normally implanted placentas (relative risk = 0.36, 95% confidence interval 0.20-0.64), even after controlling for parity and preterm or term delivery. In the same model, primiparity and preterm delivery were each associated with a doubling of risk for hypertensive disorders, regardless of the placental implantation site. Thus, regardless of parity and preterm or term delivery, placenta previa and hypertensive disorders are inversely related.