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Do lower blood pressure (BP) cut-offs in pregnancy identify women at greater risk of adverse maternal and perinatal outcome?


Department of Women's Health

, Applications accepted all year round Funded PhD Project (Students Worldwide)

About the Project

Measuring blood pressure in pregnancy is a key part of antenatal care, because high blood pressure is associated with an increased risk of problems for mothers and babies . High blood pressure in pregnancy is diagnosed when the systolic blood pressure reading reaches 140 or more, or the dyastolic number reaches 90. However, it has been suggested that use of a cut-off of 130/80 or even just systolic number of 120-129 would identify more mothers and babies at risk.

Aims/Hypotheses

To compare the risk of adverse pregnancy outcomes according to BP level, as a continuous variable and according to the American Heart Association (AHA)/American College of Cardiology (ACC) criteria: normal (<120/80mmHg), pre-hypertension (120-129/<80mmHg), stage 1 hypertension (130-139/80-89mmHg), and stage 2 hypertension (≥140/90mmHg).

Background: Clinical practice guidelines define hypertension as a systolic blood pressure (BP) ≥140mmHg or a diastolic BP ≥90mmHg. However, there is a continuous relationship between higher BP and worse maternal outcomes among women with any type of pregnancy hypertension, as well as those with pre-eclampsia. Severe elevations of BP are also associated with adverse maternal and perinatal outcomes. While a cut-off of 140/90mmHg is consistent with how hypertension is generally defined outside pregnancy, the American College of Cardiology/American Heart Association (ACC/AHA) have lowered their threshold for diagnosis to 130/80mmHg, with 130-139/80-89mmHg designated as stage 1 hypertension and ≥140/90mmHg as stage 2. The American College of Obstetricians and Gynecologists (ACOG) has not yet adopted a 130/80mmHg threshold, however, there is a literature emerging on the relationship between a threshold of 130/80mmHg for diagnosing hypertension in pregnancy and adverse outcomes. Evidence suggests that more women would be identified who have a heightened risk of pre-eclampsia, preterm birth, and gestational diabetes, with the risk being intermediate between those with BP <130/80 mmHg (defined as normal if <120/80mmHg and ‘elevated’ if systolic is 120-129) and stage 2 chronic hypertension (BP ≥140/90mmHg). Furthermore, in addition to being at increased risk of pre-eclampsia, it seems that women with stage 1 hypertension (130-139/80-89mmHg) would benefit from low-dose aspirin for pre-eclampsia prevention, based on a secondary analysis of trial data.

Methods

To understand the impact of lower BP thresholds to define hypertension in pregnancy, the candidate will have access to the following data:
(1) office BP values in primarily White British pregnant women:
Avon Longitudinal Study of Parents and Children (ALSPAC) (http://www.bristol.ac.uk/alspac/)
Southampton Women’s Survey (SWS)
(2) office BP values in ethnically- and sociodemographically-diverse pregnancy:
The early-LIfe data cross-LInkage in Research [eLIXIR] platform (https://www.guysandstthomasbrc.nihr.ac.uk/our-work/research-themes/women-and-childrens-health/elixir-study/)
Born in Bradford (BiB) (https://borninbradford.nhs.uk/)
(3) home and primary health centre BP values from low- and middle-income countries:
The Community-Level Interventions for Pre-eclampsia (CLIP) trials in Pakistan, India, and Mozambique (https://pre-empt.obgyn.ubc.ca/evidence/clip-community-level-interventions-pre-eclampsia)
The Pregnancy Care Integrating translational Science, Everywhere (PRECISE) (https://precisenetwork.org/),
(4) self-monitored BP values:

Proposed INTEGRATE trial data (N=5400 hypertensive women) (funding application submitted).

Proposed Study Design

Using these prospective cohorts, study of pregnancy outcome according to highest antenatal BP value: normal (<120/80mmHg), pre-hypertension (120-129/<80mmHg), stage 1 hypertension (130-139/80-89mmHg), and stage 2 hypertension (≥140/90mmHg, diagnosis of chronic hypertension, or treatment with antihypertensive therapy).

Baseline characteristics of interest include: demographics, past history, and current pregnancy. Antenatal surveillance include: N and type of antenatal care outpatient visits, inpatient stays (and duration), N tests of fetal well-being (including outpatient cardiotocograms and inpatient/outpatient ultrasound scans). Maternal and perinatal complications include: pre-eclampsia, gestational diabetes mellitus, induction, mode of delivery, preterm birth, stillbirth, neonatal death, birthweight <10th or >90th centiles, neonatal care unit admission (and duration and stay >=4 hr or >=48hr).

We will compare the sensitivity and specificity, and other diagnostic test properties, of antenatal BP readings for detection of the adverse pregnancy outcomes in the Methods, above.

Impact

This project will inform the definition of pregnancy hypertension and will have implications for maternity care provision globally.

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