A systematic review of heat exposure on fetal heart rate: more evidence is urgently needed
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
Objective
Ambient heat exposure is associated with an increased risk of adverse pregnancy and birth outcomes, including pre-eclampsia, stillbirth, and preterm birth.1 However, the physiological mechanisms underlying these heat-associated effects are poorly understood. Fetal heart rate (FHR) is a noninvasive marker of fetal wellbeing, reflecting fetal autonomic nervous system homeostasis. Distinct FHR characteristics, such as an increased baseline and reduced variability, are associated with fetal hypoxaemia and fetal distress, thus providing insight into fetal wellbeing.2 It is plausible that heat exposure alters uteroplacental blood flow, resulting in fetal circulatory and FHR changes, which, in susceptible fetuses, could lead to stillbirth or trigger preterm birth.
We conducted this systematic review to establish whether maternal heat exposure influences fetal physiology through mechanisms detectable via FHR.
Study Design
A systematic review according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines was conducted using Medical Subject Headings terms for ‘fetal heart rate’, ‘cardiotocography’, ‘temperature’, and ‘heat’ across MEDLINE, Embase, Global Health, CINAHL, and Cochrane Register. The research protocol was registered with PROSPERO (CRD420251034926) prior to extraction. Eligible studies measured antepartum FHR in response to observational or experimental ambient heat exposure in healthy pregnant women. To ensure FHR results were attributable to heat, studies of intrapartum FHR and women with existing co-morbidities or pyrexia were excluded.
Heat exposure was defined as any continuous measure of temperature and humidity, or heat stress index. FHR characteristics were categorized and extracted according to standard FHR parameters (baseline, variability, accelerations, and decelerations).3 Other maternal and fetal physiological parameters were extracted according to availability. Study screening and data extraction were conducted independently by 3 reviewers (J.M.A., I.M.B., and G.R.), and conflicts were resolved by a third reviewer (T.I.). Included studies were assessed using the Risk of Bias in Non-Randomised Studies-of Exposures tool. Results were narratively synthesized according to SWiM guidelines.4
Results
Our literature search identified 115 studies. After removing duplicates, 75 unique studies were screened, and 12 studies were selected for full-text review. Four studies were included in the final systematic review. The selection process is presented in Figure and the Supplementary Materials.
Ambient heat exposure is associated with an increased risk of adverse pregnancy and birth outcomes, including pre-eclampsia, stillbirth, and preterm birth.1 However, the physiological mechanisms underlying these heat-associated effects are poorly understood. Fetal heart rate (FHR) is a noninvasive marker of fetal wellbeing, reflecting fetal autonomic nervous system homeostasis. Distinct FHR characteristics, such as an increased baseline and reduced variability, are associated with fetal hypoxaemia and fetal distress, thus providing insight into fetal wellbeing.2 It is plausible that heat exposure alters uteroplacental blood flow, resulting in fetal circulatory and FHR changes, which, in susceptible fetuses, could lead to stillbirth or trigger preterm birth.
We conducted this systematic review to establish whether maternal heat exposure influences fetal physiology through mechanisms detectable via FHR.
Study Design
A systematic review according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines was conducted using Medical Subject Headings terms for ‘fetal heart rate’, ‘cardiotocography’, ‘temperature’, and ‘heat’ across MEDLINE, Embase, Global Health, CINAHL, and Cochrane Register. The research protocol was registered with PROSPERO (CRD420251034926) prior to extraction. Eligible studies measured antepartum FHR in response to observational or experimental ambient heat exposure in healthy pregnant women. To ensure FHR results were attributable to heat, studies of intrapartum FHR and women with existing co-morbidities or pyrexia were excluded.
Heat exposure was defined as any continuous measure of temperature and humidity, or heat stress index. FHR characteristics were categorized and extracted according to standard FHR parameters (baseline, variability, accelerations, and decelerations).3 Other maternal and fetal physiological parameters were extracted according to availability. Study screening and data extraction were conducted independently by 3 reviewers (J.M.A., I.M.B., and G.R.), and conflicts were resolved by a third reviewer (T.I.). Included studies were assessed using the Risk of Bias in Non-Randomised Studies-of Exposures tool. Results were narratively synthesized according to SWiM guidelines.4
Results
Our literature search identified 115 studies. After removing duplicates, 75 unique studies were screened, and 12 studies were selected for full-text review. Four studies were included in the final systematic review. The selection process is presented in Figure and the Supplementary Materials.
Date Issued
2026-09-01
Date Acceptance
2026-06-27
Citation
American Journal of Obstetrics and Gynecology MFM, 2026, 8 (9)
ISSN
2589-9333
Publisher
Elsevier
Journal / Book Title
American Journal of Obstetrics and Gynecology MFM
Volume
8
Issue
9
Copyright Statement
© 2026 TheAuthor(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
License URL
Identifier
https://www.ncbi.nlm.nih.gov/pubmed/42398827
PII: S2589-9333(26)00144-8
Publication Status
Published
Coverage Spatial
United States
Article Number
102045
Date Publish Online
2026-07-03