Helping women to keep their blood pressure at normal levels could reduce their risk of experiencing pregnancy complications, according to a new study led by researchers at the University of Bristol. The research is published in BMC Medicine today [14 January].

Fernanda Morales-Berstein, Research Associate at the University of Bristol and the study’s lead author, said: “Our findings suggest that higher maternal blood pressure increases the risk of multiple adverse pregnancy outcomes, including preterm delivery, giving birth to smaller babies, needing to have labour induced, gestational diabetes and the baby needing to be admitted to a neonatal intensive care unit.”

Maria Magnus, Senior Researcher at the Centre for Fertility and Health at the Norwegian Institute of Health and one of the study authors, added “As obesity and age at delivery continue to rise, the number of women of reproductive age with high blood pressure is increasing. High blood pressure is a common medical problem encountered in pregnancy with approximately 1 in 10 pregnant women affected.”

Several studies have shown that women with high blood pressure during pregnancy, including those who develop gestational hypertension and preeclampsia, and their babies may be at greater risk of health problems in the perinatal period. However, these studies have been mostly observational and were not designed to separate correlation from causation. If not fully accounted for, factors that influence both blood pressure and pregnancy outcomes, like socioeconomic position and body weight, can bias such analyses, resulting in misleading findings.

Although there have been trials on reducing blood pressure in pregnancy, they have generally been too small to reveal clear effects on certain complications for mothers or their babies.

Mendelian randomization is a method that uses genetic data and is less prone to biases that typically affect conventional observational studies. It has been used previously to assess the effects of maternal blood pressure on offspring birthweight and gestational duration. However, results from these studies have been inconsistent, and many other relevant outcomes have not been explored, highlighting the need for further research.

To address these research gaps, researchers at the MRC Integrative Epidemiology Unit at the University of Bristol and the Norwegian Institute of Public Health carried out the largest Mendelian randomization study of its kind, assessing the link between maternal blood pressure and 24 adverse pregnancy outcomes.

Using data from over 700,000 pregnant women, the research team found that higher blood pressure in mothers increases the risk of many maternal and fetal complications during pregnancy. For example, a 10 mmHg higher systolic blood pressure increased the risk of needing labour induction by 11% and of preterm delivery by 12%.

Deborah Lawlor, Professor of Epidemiology at the University of Bristol and one of the study authors, explained: “Our study addressed key limitations of existing conventional observational studies, which are affected by confounding, and trials, which are generally small and underpowered. We compared our results to those from other methods that rely on different assumptions. When they confirmed our main results, it increased our confidence that what we found was likely causal.”

Carolina Borges, Associate Professor in Aetiological Epidemiology at the University of Bristol, and joint last author on the paper, added: “By using genetic information to better isolate cause and effect, our study helps clarify whether a mother’s blood pressure itself contributes to pregnancy and newborn complications. This matters for clinical care and public health, as it strengthens the evidence base needed to guide prevention, monitoring, and treatment strategies aimed at improving maternal and infant outcomes.”

The authors suggest that the findings of this study should inform targeted policies aiming to improve maternal and child health in the UK and beyond. They emphasise, however, that further research is needed in non-European populations, and that clinical trials are crucial to determine the optimal timing, types of medication, and doses required to prevent pregnancy complications linked to elevated maternal blood pressure.

This research was funded by the Wellcome Trust, Medical Research Council, National Institute for Health and Care Research, British Heart Foundation, European Union and Research Council of Norway.