Association between extreme heat exposure and risk of preterm birth and its effect modifiers: a systematic review and meta-analysis.

backgroundExtreme heat is an intensifying environmental stressor, with pregnant women identified as a highly vulnerable population. Although prior researches have examined heat exposure and preterm birth (PTB), inconsistencies in heat definitions and limited evaluation of effect modifiers hinder clear interpretation.

objectiveTo systematically evaluate the association between extreme heat exposure (including temperature percentiles and heatwaves) and PTB risk.

methodsWe systematically searched five major databases (through June 2025) for studies on extreme heat ( ≥ 90th percentile) or heatwaves and PTB. Risk of bias was assessed using the WHO tool for epidemiologic studies. Random-effects meta-analysis estimated pooled relative risks (RRs), and subgroup analyses were conducted to evaluate potential modifiers. Certainty of evidence was evaluated using the WHO-adapted framework. (PROSPERO: CRD420251066785).

resultsIn total, 49 studies from 13 countries were included. Both short-term exposure ( ≤ 4 weeks before delivery; RR = 1.07, 95% CI: 1.05-1.09) and long-term exposure ( > 4 weeks before delivery; RR = 1.27, 95% CI: 1.18-1.36) significantly increased PTB risk. For short-term exposure, the 90th percentile threshold yielded the largest effect (RR = 1.20, 95% CI: 1.12-1.29; p < 0.001). Long-term associations were stronger in studies spanning multiple climate zones (RR = 1.42; 95%CI: 1.24-1.62; p = 0.024). Key modifiers included exposure window, geographic region, study design, and country income level. SIGNIFICANCE: Extreme heat exposure is significantly elevated PTB risk, particularly during late pregnancy and through cumulative long-term exposure. These findings emphasize the urgent need for targeted public health interventions and climate adaptation strategies to protect maternal and neonatal health in heat-vulnerable and low-resource regions. IMPACT: This study provides the first systematic review and meta-analysis that specifically applies standardized percentile-based definitions of extreme heat to examine its association with preterm birth (PTB). Crucially, it quantifies risk for both short-term and long-term exposure, finding that long-term exposure throughout pregnancy poses a significantly higher risk. By identifying specific vulnerability windows and the heightened risk in multi-climate zones, these results offer a framework for clinicians and policymakers to implement timely heat-health alerts and protective infrastructure for pregnant populations.