Pregnancy risks start climbing once the day's high passes 30.55 °C

Global warming is making extreme heat more common, and pregnant people are among the groups most vulnerable to it. Clinicians and public health planners have long suspected that hot weather raises the risk of adverse pregnancy and birth outcomes, but a practical question has gone largely unanswered: how hot is too hot? Without defined temperature thresholds, heat warnings aimed at pregnant people are guesswork. This review set out to synthesise the evidence on ambient heat during pregnancy and, crucially, to identify the temperatures at which the risk of adverse outcomes begins to climb.

The researchers searched five databases in June 2024 for peer-reviewed studies reporting thresholds for ambient heat in pregnancy, excluding heat from medical devices, saunas and pools, along with studies on conception, IVF and postnatal health. Risk of bias was assessed with an established assessment tool, and findings were synthesised narratively by outcome and by the way each study measured heat exposure. For the headline figures, a random effects meta-regression pooled 21 studies covering 5,589,862 pregnancies to estimate the daily maximum temperatures associated with given increases in risk. The broader narrative synthesis drew on 103 studies covering 41,951,950 people.

The pooled analysis put the daily maximum temperature associated with a 5% increase in the risk of adverse pregnancy and birth outcomes at 30.55 °C. Risk kept rising with heat: a 10% increase corresponded to 34.11 °C and a 20% increase to 39.07 °C. The narrative synthesis of 103 studies found that the field measures heat in highly inconsistent ways, with heterogeneous indicators and no comprehensive heat stress indices in routine use. The authors are open that the evidence is limited by variability between studies and by review process limitations, including no independent dual data extraction and quality assessment performed by a single person.

This work extends a scattered literature by attempting something most reviews have not: putting actual numbers on the temperatures at which pregnancy risk rises. The direction of the finding, that more heat means more risk, is consistent with what earlier studies and heat physiology would predict, so the contribution here is the thresholds rather than the association itself. The wide range of outcome definitions and heat metrics across the underlying studies means the exact figures should be treated as estimates. Open questions remain about which specific outcomes drive the risk, how humidity and exposure duration modify it, and whether thresholds differ across climates and regions.

If you are pregnant or planning to be, this is a reason to treat hot weather as a genuine health exposure rather than a comfort issue. The data suggest risk begins to rise at temperatures many places now reach routinely in summer, and climbs more steeply as conditions get hotter. None of this says a single hot day causes harm in any individual pregnancy; these are population-level associations built from millions of records. It is information worth having when deciding how to plan activity, travel and time outdoors during periods of extreme heat.