Blood Pressure Cut-Offs at 11-13 Weeks’ Gestation and Risk of Preeclampsia

BackgroundA parallel has been drawn between first trimester placental vascular maturation and maternal cardiovascular adaptations, including blood pressure. Although 140/90 mmHg is well-accepted as the threshold for chronic hypertension in the general obstetric population in early pregnancy, a different threshold could apply to stratify risk of adverse outcomes, such as preeclampsia. This could have implications for interventions, such as the threshold for initiation of antihypertensive therapy and the target blood pressure level.ObjectiveWe evaluated the relationship between various blood pressure cut-offs at 11-13 weeks’ gestation and development of preeclampsia, overall and according to key maternal characteristics.Study DesignThis secondary analysis was of data from a prospective non-intervention cohort study of singleton pregnancies delivering at ≥24 weeks, without major anomalies, at two UK maternity hospitals, 2006-2020. BP at 11-13 weeks’ gestation was classified according to American College of Cardiology/American Heart Association categories (mmHg), as: ‘Normal blood pressure’ (systolic <120 and diastolic <80), ‘Elevated blood pressure’ (systolic ≥120 and diastolic <80), ‘Stage 1 hypertension’ (systolic ≥130 or diastolic 80-89), and ‘Stage 2 hypertension (systolic ≥140 or diastolic ≥90). For blood pressure category thresholds and the outcome of preeclampsia, the following were calculated overall and across maternal age, body mass index, ethnicity, method of conception, and previous pregnancy history: detection rate, screen positive rate, and positive and negative likelihood ratios, with 95% confidence intervals (CIs). A p value <0.05 was considered significant.ResultsThere were 137,458 pregnancies screened at 11-13 weeks’ gestation. The population was ethnically diverse, with 15.9% of Black ethnicity, 6.7% of South or East Asian ethnicity, and 2.7% of mixed ethnicity, with the remainder of White ethnicity. Compared with ‘Normal blood pressure’, ‘Stage 2 hypertension’ was associated with both preterm preeclampsia (0.3 to 4.9%) and term PE (1.0 to 8.3%). A blood pressure threshold of 140/90 mmHg was good at identifying women at increased risk of preeclampsia, overall (positive likelihood ratio 5.61, 95% CI 5.14-6.11) and across maternal characteristics, compared with ’Elevated blood pressure’ (positive likelihood ratio 1.70, 95% CI 1.63-1.77) and ‘Stage 1 hypertension’ (positive likelihood ratio 2.68, 95% CI 2.58-2.77). There were two exceptions: a blood pressure threshold of 130/80 mmHg was better for the 2.1% of women with body mass index <18.5kg/m2 (positive likelihood ratio 5.13, 95% CI 3.22-8.16), and a threshold of 135/85 mmHg better for the 50.4% of parous women without a history of preeclampsia (positive likelihood ratio 5.24, 95% CI 4.77-5.77). There was no blood pressure threshold below which reassurance could be provided against development of preeclampsia (all negative likelihood ratios ≥0.20).ConclusionsThe traditional blood pressure threshold of 140/90 mmHg performs well to identify women at increased risk of preeclampsia. Women who are underweight or parous with no prior history of preeclampsia may be better identified by lower thresholds; however a randomised trial would be necessary to determine any benefits of such an approach if antihypertensive therapy were also administered at this threshold. No blood pressure threshold reassured against development of preeclampsia, regardless of maternal characteristics.