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

BACKGROUND: Extreme 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. OBJECTIVE: To systematically evaluate the association between extreme heat exposure (including temperature percentiles and heatwaves) and PTB risk. METHODS: We 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). RESULTS: In 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.