A parallel has been drawn between first-trimester placental vascular maturation and maternal cardiovascular adaptations, including blood pressure. Although 140/90 mm Hg is well-accepted as the threshold for chronic hypertension in the general obstetric population in early pregnancy, a different threshold could apply to stratify the 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.

We evaluated the relationship between various blood pressure cutoffs at 11–13 weeks of gestation and the development of preeclampsia, overall and according to key maternal characteristics.

This secondary analysis was of data from a prospective nonintervention cohort study of singleton pregnancies delivering at ≥24 weeks, without major anomalies, at 2 United Kingdom maternity hospitals, 2006–2020. Blood pressure at 11–13 weeks of gestation was classified according to American College of Cardiology/American Heart Association categories (mm Hg) as (1) normal blood pressure (systolic <120 and diastolic <80), (2) 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. A P value of <.05 was considered significant.

There were 137,458 pregnancies screened at 11–13 weeks of 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 preeclampsia (1.0% to 8.3%). A blood pressure threshold of 140/90 mm Hg was good at identifying women at increased risk of preeclampsia overall (positive likelihood ratio, 5.61 [95% confidence interval, 5.14–6.11]) and across maternal characteristics, compared with elevated blood pressure (positive likelihood ratio, 1.70 [95% confidence interval, 1.63–1.77]) and stage 1 hypertension (positive likelihood ratio, 2.68 [95% confidence interval, 2.58–2.77]). There were 2 exceptions: a blood pressure threshold of 130/80 mm Hg was better for the 2.1% of women with body mass index <18.5 kg/m² (positive likelihood ratio, 5.13 [95% confidence interval, 3.22–8.16]), and a threshold of 135/85 mm Hg better for the 50.4% of parous women without a history of preeclampsia (positive likelihood ratio, 5.24, [95% confidence interval, 4.77–5.77]). There was no blood pressure threshold below which reassurance could be provided against the development of preeclampsia (all-negative likelihood ratios ≥0.20).

The traditional blood pressure threshold of 140/90 mm Hg 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 randomized 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 is reassured against the development of preeclampsia, regardless of maternal characteristics.