A systematic review and meta-analysis on heat thresholds for maternal health and birth outcomes

Global warming poses major public health challenges, with pregnant individuals particularly vulnerable to heat. We aim to synthesise evidence on the effects of ambient heat on pregnancy and birth outcomes and identify heat thresholds associated with increased risk of these adverse outcomes. We searched Medline, Web of Science, Scopus, GreenFile and Cinahl (June 2024) for peer-reviewed studies reporting thresholds for ambient heat; heat from medical devices, saunas, pools or studies on conception, IVF, postnatal health were excluded. Participants within studies are limited to individuals capable of becoming pregnant, including women and those identifying differently. Risk of bias is assessed using the Office of Health Assessment and Translation tool. Findings are narratively synthesised according to outcome and exposure metric, and a random effects meta-regression is used to estimate heat thresholds for acute heat effects on adverse outcomes. Evidence is limited by heterogeneity and methodological variability among studies included, and through review process limitations, including no independent dual data extraction and single person performing quality assessment. Meta-analyses determine Tmax thresholds 30.55  °C, 34.11  °C and 39.07  °C for 5%, 10% and 20% risk increase (21 studies, sample=5,589,862), suggesting increasing risk of adverse pregnancy and birth outcomes with increasing heat. The narrative syntheses (103 studies, sample=41,951,950) identifies heterogeneous indicators and a lack of comprehensive heat stress indices. Funding was received through the European Union's Horizon Framework Programme and UKRI Innovate. PROSPERO registration: CRD42024610438.