Objective: To examine whether heat stress, measured by wet bulb globe temperature (WBGT), a comprehensive heat stress index is associated with placental abruption within 7 days.
Design: A space-time-stratified case-crossover design.
Setting: All 11 regions in Japan during the warm season (June-September) from 2011 to 2020.
Population: 6947 cases of placental abruption were registered in the Japan Perinatal Registry Network database.
Methods: Quasi-Poisson regression with distributed lag linear models assessed the association between heat stress, defined as daily maximum WBGT exceeding the 95th percentile of the regional distribution during the warm season, and daily placental abruption cases over a lag period of 0-7 days. Stratified analyses examined gestational week (preterm vs. term), hypertensive disorders of pregnancy (HDP), and small for gestational age (SGA).
Main outcome measures: Clinical diagnosed placental abruption.
Results: Heat stress was associated with an increased risk of placental abruption on lag 1 (relative risk [RR]: 1.23, 95% confidence interval [CI]: 1.11-1.39) and lower risk on lag 2 (RR: 0.84, 95% CI: 0.74-0.95), with negligible cumulative risk over lag 0-7 due to counterbalancing effects. Similar patterns were observed in preterm and term pregnancies. Individuals with HDP showed a higher risk on lag 1 (RR: 1.57, 95% CI: 1.31-1.88), as did those with SGA infants (RR: 1.47, 95% CI: 1.26-1.73).
Conclusions: Heat stress during pregnancy may increase the risk of placental abruption on the day after exposure, potentially advancing cases that might have occurred later, particularly in individuals with HDP or SGA infants.
Keywords: abruptio placenta; ambient temperature; climate change; epidemiology; global warming; heat alert; humidity; lagged effect; wet bulb globe temperature.
© 2025 The Author(s). BJOG: An International Journal of Obstetrics and Gynaecology published by John Wiley & Sons Ltd.