Objective
To determine if the relationship between blood pressure (BP) before 16 weeks’ gestation and subsequent onset of preeclampsia differs by parity, and by history of hypertensive disorders of pregnancy (HDP) in parous women.

Study design
Data from two studies were pooled. First, routinely collected clinical data from three metropolitan hospitals in Sydney, Australia (2017–2020), where BP was measured as part of routine clinical care using validated mercury-free sphygmomanometers. Second, prospectively collected research data from the INTERBIO-21st Study, conducted in six countries, investigating the epidemiology of fetal growth restriction and preterm birth, where BP was measured by dedicated research staff using an automated machine validated for use in pregnancy.

Main outcome
Adjusted odds ratios (aOR) (95% confidence interval (CI)) for the association of systolic BP (SBP), diastolic BP (DBP) and mean arterial pressure (MAP) with preeclampsia were obtained from logistic regression models. Models were adjusted for age, smoking, body mass index, previous hypertension, previous diabetes, and previous preeclampsia. Interactions for parity, and history of HDP in parous women were included.

Results
There were 14,086 pregnancies (Sydney = 11008, INTERBIO-21st = 3078) in the pooled analyses, 6914 (49 %) were parous, of which 414 (6.0 %) had a history of HDP. Nulliparous women had a higher risk of preeclampsia (2.6 %) compared with parous women (1.5 %): [aOR (95 %CI) 3.61 (2.67, 4.94)], as did parous women with a history of HDP (15.0 %) compared with no history (0.7 %) [12.70 (8.02, 20.16)]. MAP before 16 weeks’ gestation (mean [SD] 78.8[8.6] mmHg) was more strongly associated than SBP or DBP with development of preeclampsia in parous women [2.22 (1.81, 2.74)] per SD higher MAP] compared with nulliparous women [1.58 (1.34, 1.87)] (p for interaction 0.013). There were no significant differences on the effect of blood pressure on preeclampsia in parous women by history of HDP (p for interaction 0.5465).

Conclusion
The risk of preeclampsia differs according to parity and history of HDP in a previous pregnancy. Blood pressure in early pregnancy predicts preeclampsia in all groups, although more strongly associated in parous than nulliparous women, but no different in parous women by history of HDP.