Heat wave exposure in final gestational weeks linked to preterm birth

Key takeaways:

• Pregnant women exposed to heat waves had a higher preterm birth risk vs. unexposed women.
• Daytime-only heat wave exposure was linked to higher preterm birth risk in rural vs. urban areas.

Heat wave exposure in the final weeks of pregnancy was associated with preterm birth, especially for pregnant women in rural regions who had exposure to daytime-only heat waves, researchers in China reported.

“Research findings about urban-rural disparities in heat-related [preterm birth] are limited and inconsistent, with little such evidence available in China,” Yafei Guo, MS, from the National Key Laboratory of Intelligent Tracking and Forecasting for Infectious Diseases at the National Institute for Communicable Disease Control and Prevention and the Key Laboratory of Environment and Population Health at the National Institute of Environment Health at the Chinese Center for Disease Control and Prevention, Beijing, and colleagues wrote. “It is important to assess urban-rural and socioeconomic region-specific associations between heat waves and [preterm birth], because the findings could provide critical evidence for developing heat adaptation policies to guide clinical practice, to undertake community health education and to prevent [preterm birth].”

Guo and colleagues conducted a case-crossover study, published in JAMA Network Open, utilizing nationwide representative birth data between January 2012 and December 2019. Researchers identified 5,446,088 singleton births in 30 Chinese provinces from China’s National Maternal Near Miss Surveillance System.

Researchers categorized heat waves into three distinct types: daytime only (only daily maximum temperature exceeds thresholds), nighttime only (only daily minimum temperature exceeds thresholds) and compound (both daily maximum and minimum temperature exceeds thresholds) heat waves. Researchers also utilized six indexes including 75th and 90th percentiles of daily temperature thresholds for 2 or more, 3 or more, or 4 or more consecutive days.

Overall, 310,384 births were preterm, with a mean maternal age of 29.5 years. Pregnant women who were exposed to compound heat waves in the last week before delivery had a higher risk for preterm birth compared with unexposed women, with adjusted ORs ranging from 1.02 to 1.04 depending on the specific heat index. This association strengthened when temperature thresholds increased and lasted longer.

Pregnant women who were exposed to daytime-only heat waves had adjusted ORs for preterm birth ranging from 1.03 to 1.04 in the 75th percentile of daily temperature thresholds for 4 or more consecutive days and in the 90th percentile for 2 or more, 3 or more and 4 or more consecutive days.

Compared with urban areas, daytime-only heat wave exposure in rural areas was associated with a higher preterm birth risk. Compound heat wave exposure in higher indexes in urban areas was associated with higher preterm birth risk compared with rural areas.

Researchers observed no associations between nighttime-only heat wave exposure and preterm birth.

“These results add to the growing evidence identifying extreme heat as an essential factor associated with [preterm birth] in the context of climate change and may also be useful for urban-rural prevention strategies, protecting pregnant women from heat waves to decrease heat-related [preterm birth] in China,” the researchers wrote.