Chronic hypertension and risk of preterm delivery: National Longitudinal Study of Adolescents to Adult Health

Background: Chronic hypertension during pregnancy is associated with increased risk of adverse birth outcomes. In 2017, the American College of Cardiology and American Heart Association (ACC/AHA) lowered thresholds to classify hypertension in non-pregnant adults to SBP ≥ 130 mmHg and DBP ≥ 80 mmHg (ie stage I hypertension), resulting in an additional 4.5-million reproductive-aged women meeting criteria for hypertension. Little is known about effects of pre-pregnancy blood pressure (BP) in this range.

Objectives: To examine the effect of pre-pregnancy maternal BP on preterm delivery.

Methods: We analysed the data from two waves of the National Longitudinal Study of Adolescent to Adult Health, including participants that had measured BP at Wave IV (2008-09) and a pregnancy that resulted in a singleton live birth between Waves IV and V (2016-18; n = 2038). We categorised BP using ACC/AHA cut-offs: normal (SBP < 120 mmHg and DBP < 80 mmHg), elevated (SBP 120-129 mmHg and DBP < 80 mmHg), hypertension stage I (SBP 130-139 mmHg or DBP 80-89 mmHg) and hypertension stage II (SBP ≥ 140 mmHg or DBP ≥ 90 mmHg). We estimated risk ratios (RR) with log-binomial regression adjusting for maternal demographics, anthropometrics and medication use.

Results: The prevalence of preterm delivery was 12.6%. A standard deviation (SD) increment in SBP (SD = 12.2 mmHg) and DBP (SD = 9.3 mmHg) was associated with a 14% (95% confidence interval [CI] 2, 27) and 20% (95% CI 4, 37) higher risk of preterm delivery. Compared to normotensive controls, stage I (RR 1.33, 95% CI 1.01, 1.74) and stage II (RR 1.34, 95% CI 0.89, 2.00) hypertension were associated with increased risk.

Conclusions: We observed greater risk of preterm delivery among women with higher pre-pregnancy BP. Women with stage I hypertension during pregnancy may benefit from increased BP monitoring. Additional studies on the utility of foetal surveillance in this group are warranted.