Challenging the definition of hypertension in pregnancy: a retrospective cohort study

BACKGROUND: In routine antenatal care, blood pressure is used as a screening tool for preeclampsia and its associated adverse outcomes. As such women with a blood pressure greater than 140/90 mm Hg undergo further investigation and closer follow-up, whereas those with lower blood pressures receive no additional care. In the nonpregnant setting, the American College of Cardiology now endorses lower hypertensive thresholds and it remains unclear whether these lower thresholds should also be considered in pregnancy. OBJECTIVE: (1) To examine the association between lower blood pressure thresholds (as per the American College of Cardiology guidelines) and pregnancy outcomes and (2) to determine whether there is a continuous relationship between blood pressure and pregnancy outcomes and identify the point of a change at which blood pressure is associated with an increased risk of such outcomes. STUDY DESIGN: This was a retrospective study of singleton pregnancies at Monash Health, Australia. Data were obtained with regards to maternal characteristics and blood pressure measurements at varying gestational ages. Blood pressures were then categorized as (1) mean arterial pressure and (2) normal, elevated, stage 1 and stage 2 hypertension, as per the American College of Cardiology guidelines. Multivariable regression analysis was performed to identify associations between blood pressure categories and pregnancy outcomes. RESULTS: This study included 18,243 singleton pregnancies. We demonstrated a positive dose-response relationship between mean arterial pressure and the development of preeclampsia in later pregnancy. Across all gestational ages, the risk of preeclampsia was greater in those with "elevated blood pressure" and "stage 1 hypertension" in comparison with the normotensive group (adjusted risk ratio; 2.45, 95% confidence interval, 1.74-3.44 and adjusted risk ratio, 6.60; 95% confidence interval, 4.98-8.73 respectively, at 34-36 weeks' gestation). There was also an association between stage 1 hypertension, preterm birth, and adverse perinatal outcomes. CONCLUSION: This study demonstrated that preeclampsia and the associated adverse outcomes are not exclusive to those with blood pressures greater than 140/90 mm Hg. As such, those with prehypertensive blood pressures may also benefit from closer monitoring. Further research is essential to determine whether lowering the blood pressure threshold in pregnancy would improve detection and outcomes.