Exposure to high ambient temperatures near the time of delivery has been associated with adverse birth outcomes, but studies examining the impact on immediate newborn health remain limited. We used a time-stratified case-crossover design combined with a distributed lag nonlinear model to evaluate the short-term effects of ambient heat (0-1 day lag) on low 5-minute APGAR score (≤7; sub-categories: 6-7, 3-5, 0-2). Cases of low APGAR score among low-risk births (n=34,980) in São Paulo state (274 municipalities), 2013-2019, were extracted from Brazil’s Live Birth Information System (Sistema de Informações Sobre Nascidos Vivos). Municipality-level daily mean temperatures were constructed from ERA5-Land reanalysis data and linked with case and control days by date and municipality of delivery. Models were adjusted for relative humidity and stratified by maternal age, race/ethnicity, education, parity, timing of prenatal care initiation, infant sex, municipality-level deprivation, and Köppen climate zone. Overall, exposure to high (95th percentile: 26.1°C) versus moderate (50th percentile: 20.9°C) temperature 0-1 days before delivery was associated with 8% higher odds (OR: 1.08, 95% CI: 1.02-1.14) of low APGAR score (≤7). In stratified analyses, heat-associated risks were elevated among infants born to women with <12 years of schooling (1.10, 1.03-1.17) and/or self-identifying as Brown/Parda (1.10, 1.01-1.20). Associations were primarily driven by same-day (lag 0) exposure and were only observed in newborns with moderately low APGAR scores (6-7). Acute exposure to ambient heat may adversely impact newborns’ immediate health in low-risk live-births, highlighting the need for heat mitigation measures near the time of delivery.

The authors have declared no competing interest.

N/A.

This work was supported by Wellcome Trust (https://wellcome.org/) under Grant Number 226306/A/22/Z (recipients: EBB, MLB, LS, JP, ESP).The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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