High ambient temperatures and hypertensive disorders of pregnancy: a systematic review

Introduction

Exposure to high ambient temperatures (heat), is consistently associated with adverse birth outcomes, particularly preterm birth, however, evidence for maternal outcomes, including hypertensive disorders of pregnancy (HDP), remains unclear. HDP are a leading cause of maternal mortality and morbidity, whose aetiology is poorly understood. In the context of climate change, clarifying this potential association is of global health importance.

Methods

We searched PubMed, EMBASE and Web of Science (inception to 23rd September 2025), for studies examining associations between heat and HDP (gestational hypertension, pre-eclampsia and eclampsia). Two reviewers independently screened, extracted data and assessed risk of bias. A narrative synthesis was completed.

Results

Thirty-six studies (14 cohort, 1 case-control and 21 ecological time-series; published 1965-2025) from 3,503 records screened. Together, these included over 8.3 million pregnancies, 86,065 cases of gestational hypertension/any HDP, 190,587 cases of pre-eclampsia and 2,218 cases of eclampsia, across 18 countries. Ten cohorts reported increased HDP risk associated with heat in early-pregnancy. However, three reported non-significant associations and one a protective association, possibly reflecting insufficient power, inappropriate exposure windows, or poor preservation of temporality. Ecological results were heterogeneous, potentially reflecting variation in exposure windows examined (e.g., month of delivery, diagnosis, hospitalisation, or conception). Eight reported increased risk, five reduced, five non-significant, and three didn’t conduct statistical testing.

Conclusions

Cohort evidence, largely from the past decade, suggest early-pregnancy heat exposure may increase HDP risk. However, more high-quality, individual-level studies in representative populations in different climate zones are needed to clarify this association which has implications for climate action efforts.

Key messages

• Heat exposure in early pregnancy may increase HDP risk.

• Further methodologically rigorous, geographically diverse cohort studies are needed to clarify this association.