Relationship Between Third-Trimester Low Maternal Blood Pressure and Small-for-Gestational-Age Birth Weight in Pregnant Individuals With Mild Chronic Hypertension

OBJECTIVE: To estimate the association between third-trimester maternal low blood pressure (BP) and delivery of a neonate with small-for-gestational-age (SGA) birth weight in patients treated for mild chronic hypertension. METHODS: This is a secondary analysis of the CHAP (Chronic Hypertension and Pregnancy) study, which randomized pregnant participants with mild chronic hypertension to treatment to achieve goal BP below 140/90 mm Hg compared with usual care. We calculated mean systolic and diastolic BPs between 28 and 34 weeks of gestation and excluded those with systolic BP of 140 mm Hg or higher or diastolic BP of 90 mm Hg or higher. We defined low BP as mean systolic BP below 110 and mean diastolic BP below 70 mm Hg or mean arterial pressure below 80 mm Hg and compared those individuals with participants with mean systolic BP of 110–139 mm Hg or mean diastolic BP of 71–89 mm Hg or both or mean arterial pressure of 80 mm Hg or higher. Our primary outcome was delivery of a neonate with SGA birth weight (birth weight below the 5th percentile). Logistic regression estimated the association between low BP and SGA birth weight, and adjusted odds ratios (aORs) and 95% CIs were reported. RESULTS: Of 2,408 CHAP participants, 1,205 (50.0%) met analysis criteria. Of those 1,205, 31 (2.6%) had low BP and 1,174 (97.4%) had mean BP 110/70–139/89 mm Hg; 33 (2.7%) had mean arterial pressure below 80 mm Hg, and 1,172 (97.3%) had mean arterial pressure of 80 mm Hg or higher. Having a neonate with SGA birth weight below the 5th percentile occurred in 62 participants (5.1%): 1 of the 31 (3.2%) with BP below 110/70 mm Hg and 1 of the 33 (3.0%) with mean arterial pressure below 80 mm Hg. There was no significant association between delivery of a neonate with SGA birth weight less than the 5th percentile and low BP by either mean systolic BP and mean diastolic BP (aOR 0.46, 95% CI, 0.06–3.58) or mean arterial pressure (aOR 0.53, 95% CI, 0.07–4.01). We found a nonlinear relationship between mean arterial pressure and delivery of a neonate with SGA birth weight less than the 5th percentile, and, as mean arterial pressure decreased, there was lower probability of having a neonate with SGA birth weight (P5.02). CONCLUSION: Pharmacologic treatment of mild chronic hypertension infrequently results in low BP and does not appear to be associated with delivery of a neonate with SGA birth weight less than the 5th percentile for birth weight.