BACKGROUND: Pregnant women are vulnerable to extreme heat due to physiological adaptations that alter thermoregulation and metabolism; however, the underlying physiological mechanisms remain inconclusive. This systematic review and meta-analysis synthesises the effects of environmental heat on maternal, placental, and fetal physiology, identifying key biophysiological pathways. METHODS: We searched PubMed and Embase from database inception to Jan 13, 2025, for studies of pregnant women exposed to heat. Observational and experimental studies conducted in laboratory or field settings were included. Grey literature was not included. Alterations in typical physiological functions resulting from heat exposure across maternal, placental, or fetal responses were considered outcomes. AB and AAMQ screened and extracted data independently. Random-effects meta-analyses and meta-regression were applied to examine associations between heat stress exposure parameters and physiological responses. The study was registered with PROSPERO (CRD42024511153). FINDINGS: 91%; seven studies), 0·5°C in maternal core temperature (0·5-0·6; 84%; 12 studies), 1·9°C in maternal skin temperature (0·4-3·4; 99%; six studies), and 13 bpm in fetal heart rate (8·8-16·5; 78%; seven studies). On meta-regression, increased core temperature was associated with higher intensity activities and increased heat stress and inversely associated with gestational age. There were inconsistent changes in uterine and umbilical artery blood flow. However, a higher susceptibility to impaired uteroplacental blood flow was observed among pregnant women with hypertension or pre-eclampsia. INTERPRETATION: We found consistent acute physiological stress responses to heat exposure, with evidence that these responses were worse in pregnant women with pre-eclampsia. The paucity of available data highlights the need for more research to identify the underlying biological mechanisms that mediate pregnancy outcomes with high heat exposure. FUNDING: Wellcome Trust.
Bonell et al. (Fri,) studied this question.