Relative high heat exposure increases the likelihood of adverse birth outcomes: Multi-country evidence of heat impacts on birth outcomes

Extreme high temperatures pose a growing threat to perinatal health, which is expected to intensify with accelerating climate change. While there is a growing body of evidence documenting heat-perinatal health outcomes, comparing studies worldwide poses multiple challenges, including heterogeneity in study designs and heat metrics. Furthermore, evidence from low-and-middle income countries is particularly underrepresented in the global evidence base. Using over 5 million births from hospital (11 locations), regional (two locations) and national (three countries) registry data across Africa, Europe and Latin America and the Caribbean (LAC), we employed a time-stratified case-crossover design to evaluate the association between short-term (<7 days) heat exposure comparing different heat metrics (temperature, Wet Bulb Globe Temperature, Universal Thermal Climate Index (UTCI), and heat index) and birth outcomes (preterm birth [PTB], stillbirth and low Apgar score). Here we show that compared to the 75th percentile of maximum UTCI, the 99th percentile was associated with 1.10-fold higher odds of PTB [95%CI: 1.07-1.14], with stronger heat-exposure associations in hospital-based registries in Africa (OR=1.22 [95%CI:1.05-1.42]) and LAC (OR=1.27 [95%CI :0.90-1.79]) than population-based registries in Europe (OR=1.10 [95%CI: 1.08-1.11]). We observed a 1.09-fold increase in the odds of stillbirth [95%CI: 0.92-1.27] and 1.08-fold increase in low Apgar score [95%CI: 0.99-1.18], with higher risks in Africa compared to Europe. All heat exposure metrics were highly correlated, with similar effect estimates observed, but slightly higher effects using UTCI in more humid countries. As one of the largest studies using harmonised statistical approaches, we provide robust estimates of the detrimental associations of heat on multiple birth outcomes, with stronger associations in low- and middle-income settings, highlighting the disproportionate vulnerability of these populations.