Trimester-specific exposure to multiple heat indicators and adverse birth outcomes across four European countries

The health of pregnant women and their fetuses is increasingly challenged by abnormal heat events. Few studies have disentangled the trimester-specific effects of heat exposure during pregnancy on perinatal outcomes and examined potential differences across multiple heat indicators. This study included singleton births from multiple national and region registries across Europe: Sweden (2014-2019), Belgium (2012-2022), Italy (Lazio Region) (2001-2019), and Greece (1999-2021). We obtained 2-m air temperature, Universal Thermal Climate Index (UTCI), Wet Bulb Globe Temperature (WBGT), and Heat Index data from the ERA5 reanalysis dataset. Our study outcomes were preterm birth (PTB), stillbirth (SB), small for gestational age (SGA), and large for gestational age (LGA). Cox proportional hazard models, with gestational age as the underlying time scale, were applied to evaluate trimester-specific associations between multiple heat indicators and adverse birth outcomes. Country-specific estimates were then combined through meta-analyses. In total, 4,924,422 mother-child pairs were included in the analysis. An increase in mean temperature from the 75th to the 99th percentile in the third trimester was associated with PTB (HR = 1.51, 95% CI: 1.34-1.71), SB (HR = 1.54, 95% CI: 1.44-1.65), SGA (HR = 1.16, 95% CI: 1.11-1.20), and LGA (HR = 1.12, 95% CI: 1.07-1.17). Comparable associations were observed across heat indicators and countries for each trimester. Our findings suggest that the third trimester is a critical exposure window of vulnerability for heat during pregnancy. The comparable effect estimates among heat indicators highlight the usefulness of mean air temperature in Europe.