Chronic hypertension was defined as elevated blood pressure of 140/90 mm Hg or higher prior to pregnancy or before 20 weeks of gestation not attributable to gestational trophoblastic disease(10). Pregnant women carrying multiple or anomalous fetuses or pregnancies with incomplete data were excluded. Data on patients’ characteristics including age, parity, body mass index (BMI), onset of hypertension, end organ involvement, prior hypertensive disorders complicating pregnancy, smoking before or during pregnancy, level of blood pressure at booking and during pregnancy, number of antihypertensive agents use before pregnancy, number of prenatal visits and coexisting medical diseases were collected. The primary outcome was the incidence of superimposed preeclampsia on chronic hypertension which diagnosed by a new-onset presenting of proteinuria of 300 mg or greater in 24 hours in hypertensive women with absence proteinuria before 20 weeks of gestation(10). In addition, superimposed preeclampsia on chronic hypertension would be made if a sudden increase in proteinuria or blood pressure or platelet counts less than 100,000/μL in women with hypertension and proteinuria before 20 weeks of gestation occurred(10). The following maternal and neonatal outcomes were determined: gestational age at delivery, mode of delivery, placental abruption, HELLP syndrome (hemolysis, elevated liver enzymes