Heat stress and maternal complications in India: A nationwide analysis using the universal thermal climate index

Ambient heat is rising in India and may affect maternal health during pregnancy. Physiological adaptations in pregnancy heighten heat strain, yet national evidence examining exposure using Universal Thermal Climate Index (UTCI) is limited. We linked National Family Health Survey-5 records for the most recent live birth (2019-2021), with district-level UTCI. Maternal Complications were using four self-reported pregnancy complication indicators. Heat exposures metrics included mean UTCI and mean of daily maximum UTCI calculated for full gestational period and for trimester-specific windows. Multivariable logistic regression models estimated adjusted odds ratios (aORs) per 1°C (1 K) increase in UTCI, adjusting for maternal age, parity, household wealth quintile, and place of residence, with state fixed effects. Analyses incorporated sampling weights and robust standard errors. The analytic sample comprised 176,193 women; 56.6% reported at least one maternal complication. Higher mean UTCI across pregnancy was associated with increased odds of maternal complications (aOR 1.004; 95% confidence interval [CI]: 1.000-1.008; p = 0.042), with the strongest effect in the second trimester (aOR 1.006; 95% CI: 1.002-1.010; p = 0.007); first and third trimesters were not significant. In contrast, the mean of daily maximum UTCI showed an inverse association (full pregnancy aOR 0.993; 95% CI: 0.988-0.998; p = 0.005), which was significant in the first and second trimesters. Higher parity was associated with lower odds of maternal complications, while urban residence showed marginally higher odds. Sustained gestational heat load measured with UTCI was modestly associated with maternal complications in India with a possible mid pregnancy vulnerability. Antenatal services and Heat Action Plans should incorporate heat risk screening, scheduling, cooling, and hydration, and future studies should test non-linear exposure relationships and extreme heat events.