Humid Heat Stress and Preterm Birth Risk in Brazil

Climatic conditions of humid heat stress, characterized by prolonged periods of high temperature and elevated humidity, can influence reproductive health, yet remain understudied in tropical settings. We estimated the association between exposure to combined high temperature and high relative humidity and the occurrence of preterm birth in Brazil during the warm months (January–March and October–December) of 2023. We integrated daily municipal-level meteorological data (BRDW-GD) with live birth records (SINASC), defining humid heat stress as simultaneous exposure to temperatures above 35 °C and relative humidity above 50% within a 30-day window preceding delivery. Within this window, we assessed whether pregnant individuals experienced 10, 7, 5, or 3 days under these conditions. Effects were estimated using logistic regression, adjusting for maternal and newborn characteristics, and stratified by major Brazilian regions. Results revealed substantial regional heterogeneity. Between October and December, a protective effect was observed in the Northern region, whereas the Northeast, Southeast, and Midwest regions exhibited increased risk. During the summer months (January–March), this pattern reversed: the Southeast showed a protective effect, while the North demonstrated elevated risk. At the national level, the associations were positive but of small magnitude.