HIGH Horizons - High temperature exposure in pregnancy and adverse maternal and neonatal outcomes: a systematic review and meta-analysis

This systematic review, deliverable 2.6 aims to summarise and quantify the impacts of high temperatures on maternal, foetal, and neonatal health, while considering windows of vulnerability, subgroups at risk, and potential causal pathways.
Employing similar methodologies to Chersich et al. 2019., we systematically searched Medline and Web of Science up to September 2018. Updates were conducted in September 2019 and June 2023 in Medline, supplemented by reference list screening. Eligibility criteria included epidemiological studies on high ambient temperature exposure during pregnancy. All maternal and neonatal outcomes were included. A narrative synthesis and meta-analyses were conducted, categorising outcomes clinically and effect estimates into three temperature metric groupings.
The search identified 15,559 and 4,704 records in 2019 and 2023, ultimately with 198 included articles. The 198 articles were expanded to yield 271 outcomes. A discernible surge in studies since 2012 was observed. Sixty-six countries, with high-income countries (54,1%), predominantly in global north, represented the geographical distribution. Neonatal outcomes predominated (n=115), followed by maternal (n=52), and foetal and perinatal (n=51). Among the 20 outcome themes identified, preterm birth (n=73), low birth weight (n=48), hypertensive disorders in pregnancy (n=28), congenital anomalies (n=15), and stillbirths (n=19) were the most studied. Most outcomes had a significant number of studies with a positive direction of effect, except for low birth weight, hypertensive disorders in pregnancy and congenital anomalies, where results were heterogenous. Random effects meta-analysis revealed elevated odds of preterm birth per 1°C increase in temperature of 1.04 (95%CI 1.03, 1.05) and 1.17 (95%CI 1.10, 1.23) during heatwaves. The odds of stillbirths rose 1.14 (95%CI 0.99, 1.31) per 1°C increase in temperature and 1.08 (95%CI 0.96, 1.21) at higher versus lower temperatures.in effect modification indicated higher risk among age extremes, lower socioeconomic status, Black women, and women who smoke.
This review illustrates a growing body of evidence indicating that temperature exposure during pregnancy has an adverse impact on various maternal and neonatal outcomes. Effect modification suggests impacts may be highest in low and middle-income countries, however, these is a mismatch in research distribution. Further, there are many outcomes that have suggestive evidence for heat impacts, but more research for these is required. This review demonstrates the lack of universally accepted definitions and methodologies, providing opportunities for standardisation in future research. These findings advocate for enhanced awareness, resource allocation for adaptation and mitigation efforts, and further research to understand impacts in vulnerable populations as there is substantial evidence to demonstrate impacts of global warming on pregnant women and their babies.
PROSPERO registration CRD42019140136, CRD42018118113 and CRD42020173519

This systematic review and meta-analysis have been published in Nature Medicine: Lakhoo DP, Brink N, Radebe L, Craig MH, Pham MD, Haghighi MM, Wise A, Solarin I, Luchters S, Maimela G, Chersich MF; Heat-Health Study Group; HIGH Horizons Study Group. A systematic review and meta-analysis of heat exposure impacts on maternal, fetal and neonatal health. Nat Med. 2024. doi: 10.1038/s41591-024-03395-8.