Preconception heat exposure and risk of macrosomia: evidence from a nationwide birth cohort in China.

Extreme heat is an increasing concern for maternal and neonatal health, yet the relevance of heat exposure before conception remains insufficiently understood. Using a nationwide birth cohort of 233,842 mother-infant pairs in China, including 12,737 macrosomia cases, we examined associations of preconception and prenatal extreme heat exposure with macrosomia, defined as birth weight >4000 g. Extreme heat was characterized using peak-based indicators and cumulative burden metrics across preconception windows of up to 24 months and gestational periods. Preconception heat exposure was consistently associated with higher macrosomia risk. Maximum heatwave intensity during the preconception period was associated with a 7% higher risk of macrosomia, with a magnitude comparable to that observed during pregnancy (HR = 1.07, 95% confidence interval: 1.06-1.08). Cumulative heat burden showed stronger associations over longer preconception windows; for example, cumulative heatwave intensity during the 24 months before conception was associated with a 6% higher risk of macrosomia (HR = 1.06, 95% CI: 1.05-1.07). In stratified analyses, associations between prenatal heat exposure and macrosomia were attenuated among mothers with higher preconception heat exposure, suggesting that prior thermal history may modify susceptibility to heat during pregnancy. Associations were stronger among mothers with lower educational attainment, pre-existing medical conditions, lower pre-pregnancy body mass index, and advanced maternal age; for example, the association for maximum heatwave intensity was stronger among mothers with lower education than among those with higher education (HR = 1.10 vs. 1.05). These findings suggest that heat-related risks to fetal growth may begin before pregnancy and that preconception thermal environments should be considered in climate-health risk assessment and maternal health protection.