The burden of premature births attributed to heat across 13 countries.

Background Climate change threatens global health, particularly among vulnerable populations such as pregnant individuals and their newborns. Evidence linking heat to premature birth is largely based on single-location studies or heterogeneous meta-analyses, leaving important gaps regarding underrepresented regions, preterm subgroups, and the role of maternal and infant characteristics.Objectives To quantify the association between heat and preterm birth (PTB) across multiple countries, assess gestational-age-specific effects, and identify maternal vulnerability factors.Methods We analysed 36.6 million births occurring during the warm season from 250 locations in 13 countries to assess heat effects on PTB. Distributed lag non-linear models (DLNM) with quasi-Poisson regression estimated heat-PTB associations and the fraction of PTB attributable to heat. Gestational-age subcategories (extreme, very, late, and at-term) and socio-economic vulnerability profiles were also examined.Results Overall, 1.4% (95% CI: 1.3-1.5) of PTB were attributable to heat (855 PTB per million births), with national burdens from 628 to 1,347 PTB per million. Higher susceptibility was suggested for younger, single, non-primiparous, less-educated, and socio-economically deprived mothers, and among female fetuses. Late PTB showed the largest risk; at-term births also displayed a small but consistent heat-related increase.Conclusions This large analysis of heat-related PTB using harmonized individual-level data indicates that heat increases PTB risk, with variations across countries and climates. It also shows that heat can trigger labour beyond the typical PTB window, affecting pregnancies not usually considered clinically vulnerable. Overall, these findings underscore the need for strategies to mitigate heat-related risks during pregnancy, particularly among socio-economically vulnerable populations.