Extreme temperature and preterm birth: Time windows and geographical disparities

To thoroughly explore how exposure to heat or cold influences the preterm birth (PTB) risk in different time windows.

Embase, Web of Science, PubMed and Scopus were systematically searched for studies about the association of abnormal temperature exposure and PTB risk from inception to March 2025. Corresponding data like the time window of temperature exposure, temperature, and PTB prevalence etc. were extracted. A random effects model was employed to merge effect estimates (risk ratios, RR) in the statistical analysis, stratified by temperature percentile and exposure time window, and a total of six independent meta-analyses were performed. This research was carried out and documented in line with the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guidelines.

This study incorporated 32 studies. High temperatures within one week before delivery significantly increased PTB risk (RR = 1.05, 95 % CI: 1.02-1.08), particularly in subtropical regions (RR = 1.09, 95 % CI: 1.01-1.18), developing countries (RR = 1.06, 95 %CI: 1.01-1.12), and prospective studies (RR = 1.23, 95 % CI: 1.02, 1.49). For exposure windows of 1-4 weeks before delivery, a positive association was found (RR = 1.07, 95 % CI: 1.02-1.12), with subtropical regions at highest risk (RR = 1.10, 95 % CI: 1.01-1.20). Exposure beyond four weeks before delivery also increased PTB risk (RR = 1.20, 95 % CI: 1.00-1.44). Cold exposure did not significantly increase PTB risk, but did so more than four weeks (RR = 1.21, 95 % CI: 1.05-1.39).

This study indicates that prenatal high-temperature exposure is associated with an elevated risk of preterm birth, particularly during early pregnancy and in subtropical climates. The impact of cold exposure on PTB is unclear, but highlights the need for climate-resilient prenatal care amid global warming, including heat health warnings and cooling aid. This evidence is vital for tackling climate change's health risks to mothers and infants.