- Design
- Systematic review and random-effects meta-analysis of observational studies
- Population
- 28 studies, 23,268,235 participants
- Primary outcome
- Hypertensive disorders of pregnancy, pre-eclampsia, gestational hypertension
- Effect
- Heat and HDP OR 1.58 (95% CI 1.31–1.90); pre-eclampsia OR 2.01 (1.42–2.84); per 1 °C OR 1.03 (1.02–1.04)
This meta-analysis pooled 28 observational studies covering more than 23 million pregnancies, searched to February 2026 and published on 22 August. Heat exposure was associated with hypertensive disorders of pregnancy (OR 1.58) and more strongly with pre-eclampsia (OR 2.01). Cold showed no significant pooled association.
The associations were stronger in arid climates and in late pregnancy, from 28 weeks. Modelled as a continuous exposure, each 1 °C rise carried about 3% higher odds.
The heterogeneity is extreme (I² 98–99% for the main estimates), so the pooled number is less trustworthy than the direction. For clinicians in hot regions, including much of India, it is a reason to ask about heat exposure at work and home in the third trimester, not a reason to change screening or aspirin thresholds.
- Ask women in late pregnancy about heat exposure at work and at home.
- Advise hydration, shade and rest during heatwaves, especially after 28 weeks.
- Keep home blood-pressure checks going through hot months for women already at risk.
- Do not change aspirin or screening criteria on this evidence alone.
Why it matters
In hot climates, the weather may be a pre-eclampsia risk factor that antenatal care rarely asks about.
Don't overread it
The studies were observational and very inconsistent; the pooled odds ratio is imprecise.
The statistics, in plain English
An I² of 98% means almost all the variation between studies reflects genuine differences in setting or method, not chance, so a single pooled number summarises very different results. The per-degree estimate (OR 1.03, I² 48%) is more consistent and suggests a steady rise with temperature.
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