ObjectiveTo examine the ability of five common definitions of hypertension in pregnancy to predict adverse maternal and perinatal outcomes.MethodsWe studied 9133 singleton nulliparous pregnancies with early prenatal care from the Collaborative Perinatal Project, a large cohort study conducted between 1959 and 1965. Definitions from five different groups were evaluated. Severe maternal and perinatal morbidity and mortality were used as the outcome measurements. Sensitivity, specificity, and positive predictive value for outcomes were compared across various definitions.ResultsBlood pressure alone had very poor discriminatory power to predict adverse outcomes. Positive predictive values of adverse outcomes by the diagnosis of preeclampsia were 18–20% based on antepartum and intrapartum blood pressures and 22–36% based on antepartum blood pressure only. Mild hypertension occurring for the first time in labor and isolated mild systolic hypertension were not associated with adverse outcomes. Similarly, an increase in diastolic blood pressure of 15 mmHg that did not achieve an absolute value of 90 mmHg did not predict adverse outcome.ConclusionNeither blood pressure nor blood pressure and proteinuria are accurate predictors of severe adverse maternal and perinatal outcomes. Mild hypertension occurring for the first time in labor and isolated mild systolic hypertension should not be considered indicators for hypertensive disorders in pregnancy in a research definition.