Risk of adverse birth outcomes rises with daily maximum temperature, from about 30.55 °C

Why it matters

Heat warning systems for pregnant people lack evidence-based trigger levels. These estimates give first approximations for tiered warnings. The authors stress they come mostly from cooler regions and may understate risk in hotter, poorer places.

What they did

They searched five databases in June 2024 for peer-reviewed studies that reported heat thresholds for maternal health or birth outcomes. 103 studies were included in a narrative review. For the meta-analysis, they converted effect estimates to odds ratios and fitted a random-effects quadratic meta-regression of risk against daily maximum temperature (Tmax). They used same-day (lag 0) effects from 21 studies. They also ran the model for preterm birth alone and for temperate climates alone.

Key findings

• Tmax thresholds were 30.55 °C, 34.11 °C and 39.07 °C for 5%, 10% and 20% higher risk, with 24.53 °C marking no increase.
• For preterm birth alone, the thresholds were similar: 30.05 °C, 33.34 °C and 38.64 °C.
• In temperate climate zones, thresholds were lower: 29.03 °C, 31.86 °C and 35.64 °C.
• Mean temperature and percentiles showed no meaningful link with adverse outcomes, and other heat indices had too few studies for meta-analysis.
• Findings for preeclampsia and gestational hypertension were mixed, while premature rupture of membranes more consistently showed higher risk with heat.

Limitations

• Studies come mostly from cooler, richer regions, and only four cover low-income countries, so risk in hotter places is likely underestimated.
• The meta-analysis used only same-day effects and mixed all pregnancy periods, leaving out delayed effects, exposure duration, heatwaves and minimum temperature.
• Data extraction had no independent dual check, one person did the risk-of-bias assessment, and the studies were highly varied in heat measures.

Glossary

• Tmax: The highest air temperature reached in a day.
• Meta-regression: A statistical method that pools results from many studies and relates them to a factor such as temperature.
• WBGT: Wet Bulb Globe Temperature, a heat stress index that combines air temperature, humidity, radiant heat and air movement.
• Lag 0: The effect of heat on the same day, with no delay.