Gestational-age-specific reference ranges for blood pressure in pregnancy: findings from a prospective cohort

OBJECTIVE Pregnancy is a period of considerable change in blood pressure, with an early pregnancy decrease followed by a late pregnancy rise. High blood pressure in pregnancy is associated with adverse perinatal outcomes for the mother and offspring. We aimed to define normal ranges of blood pressure across gestation. METHODS We used repeated antenatal blood pressure measurements [median (interquartile range) 10 (9-11) per woman] for 10,327 women. Multilevel models were used to derive longitudinal reference ranges for SBP and DBP from 12 to 40 weeks gestation for the whole cohort, for women with normal pregnancies (without essential hypertension or preeclampsia who delivered an appropriate-size-for-gestational age infant at term) and for subgroups of normal pregnancies defined by different levels of maternal prepregnancy BMI, smoking and parity. RESULTS In normal pregnancies, the mean (95% reference range) SBP and DBP for nulliparous women at 12 weeks gestation were 112.1 (88.6-135.5) and 65.4 (48.9-81.9) mmHg, and at 37 weeks were 116.0 (92.3-139.7) and 70.0 (52.2-87.9) mmHg, respectively. For every additional 10  mm Hg of blood pressure at 12 weeks, normal ranges were 2-3 mm Hg higher across gestation. Reference ranges for multiparous women were 1-2  mm Hg lower throughout pregnancy. Stratified reference ranges were higher for women in higher prepregnancy BMI categories, and lower for smokers than for nonsmokers throughout pregnancy. CONCLUSION Normal ranges for blood pressure vary with gestation age and by maternal subgroups. Whole population and stratified normograms could be used as a reference to identify abnormal trajectories.