Short-term ambient heat exposure and low APGAR score in newborns: A time-stratified case-crossover analysis in São Paulo state, Brazil (2013–2019)

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.