Analytical approaches to evaluating hypertensive disorders of pregnancy

Data from the A Randomized Trial of Induction Versus Expectant Management study suggested that low-risk pregnant patients randomized to expectant management at term had a higher risk for developing hypertensive disorders of pregnancy than pregnant patients randomized to elective induction at 39 weeks. In addition, hypertensive disorders of pregnancy were reported to decrease with advancing gestational age when comparing outcomes by gestational age at delivery.

This study aimed to verify these contrasting findings by evaluating the relationship between hypertensive disorders of pregnancy at term and gestational age at delivery.

This was a secondary analysis of a multicenter, prospective cohort study of nulliparous pregnant patients with singleton gestations (from the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be). Pregnant patients who delivered ≥37+0 weeks' gestation were included. Patients were excluded if they did not provide consent for data release in subsequent studies or if they had missing outcome data. The primary outcome was hypertensive disorders of pregnancy, defined as gestational hypertension or preeclampsia with or without severe features according to the American College of Obstetricians and Gynecologists guidelines. Descriptive statistics were used to evaluate hypertensive disorders of pregnancy in the following 2 ways: analysis (1), incidence of hypertensive disorders of pregnancy by gestational age week among all ongoing pregnancies and analysis (2), the incidence of hypertensive disorders of pregnancy by gestational age week among deliveries at that gestational age week. It was assumed that hypertensive disorders of pregnancy were not expectantly managed at term.

Of the 8011 pregnant patients included in this analysis, 1003 (12.5%) had hypertensive disorders of pregnancy: 162 (24.5%) delivered at 37+0 to 37+6 weeks' gestation, 232 (18%) delivered at 38+0 to 38+6 weeks, 310 (12.1%) delivered at 39+0 to 39+6 weeks, 207 (8.7%) delivered at 40+0 to 40+6 weeks, and 92 (8.1%) delivered at ≥41+0 weeks. In analysis 1, the incidence of hypertensive disorders of pregnancy increased with advancing gestational age among all ongoing pregnancies with a hypertensive disorder of pregnancy (2.0% at 37 weeks and 8.1% at 41 weeks). In analysis 2, the incidence of hypertensive disorders of pregnancy decreased with advancing gestational age among deliveries at that gestational age week alone (24.5% at 37 weeks and 8.1% at 41 weeks).

Our results confirm the findings from the A Randomized Trial of Induction Versus Expectant Management study showing that the risk for hypertensive disorders of pregnancy increases with advancing gestational age, but hypertensive disorders of pregnancy is more common among deliveries at earlier gestational ages. These key differences illustrate how important the study design and analytical approach are to accurately interpret results and apply findings clinically.