OBJECTIVE: To evaluate whether blood pressure (BP) less than 140/90 mm Hg is associated with lower risk of adverse pregnancy outcomes in women with mild chronic hypertension. METHODS: This was a secondary analysis of women with chronic hypertension diagnosed before 20 weeks of gestation (either BP 140/90 mm Hg or greater on two occasions at least four hours apart or previously diagnosed and on antihypertensive therapy) enrolled in theMaternal- Fetal Medicine Units Network's High-risk Aspirin preeclampsia prevention trial. Outcomes including a primary composite (perinatal death, severe preeclampsia, placental abruption, and indicated preterm birth less than 35 weeks of gestation) and small for gestational age (SGA) were compared by study preenrollment BP analyzed as a categorical (less than 140/90, 140-150/90-99, or 151-159/100- 109 mm Hg) and as a continuous variable. RESULTS: Among 759 women with singleton pregnancy and preenrollment BP less than 160/110 mm Hg, the incidence of the primary composite outcome (10.7%, 19.0%, 30%) and SGA (8.8%, 12.3%, 23.7%) increased with increasing BP category (P values ≤.001). The adjusted odds ratio (95% confidence interval) for the primary composite was 2.0 (1.3-3.2) for 140-150/90-99 mm Hg and 3.2 (1.6-6.3) for 151-159/100-109 mm Hg compared with BP less than 140/90 mm Hg. The results for SGA were 1.6 (0.9-2.8) and 3.8 (1.8-7.9), respectively. Models including continuous systolic and diastolic BP revealed increasing adverse outcomes per 5-mm Hg rise in diastolic but not systolic BP: primary composite (19% per 5 mm Hg) and SGA (22% per 5 mm Hg). CONCLUSION: The risks of adverse pregnancy outcomes in women with chronic hypertension are lower with preenrollment BP less than 140/90 mm Hg as compared with higher BP categories and increase with increasing BP. © 2014 by The American College of Obstetricians and Gynecologists. Published by Lippincott Williams & Wilkins.