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.