Heatwaves and Health at Birth

Draft — public (please do not cite without permission)

This paper examines the impact of prenatal exposure to extreme heat on birth outcomes in Spain. I link over 460,000 administrative birth records (2012–2021) to municipality-level, high-resolution temperature data to construct trimester-specific measures of in-utero heat exposure. Heatwaves are defined as at least three consecutive days above the municipality-specific 95th percentile, with alternative thresholds used for robustness. Unlike most existing studies, I find that heatwave exposure—especially in the third trimester—increases birth weight and gestational age and slightly raises the likelihood of cesarean delivery. The results are stable across multiple specifications and heat definitions. To explore mechanisms, I combine births with new data on local healthcare infrastructure. The positive effects of heatwaves are significantly larger in municipalities with greater access to primary care, suggesting that robust healthcare systems help mothers adapt to extreme heat. Overall, the findings highlight the importance of local resilience in shaping climate-health impacts.