Extreme Heat Tied to Preterm Birth, but ‘Greenness’ Helps

Extreme heat exposure during the third trimester of pregnancy was associated with preterm birth, but higher levels of greenness moderated its role, an Australian cohort study of more than a million singleton pregnancies found.

Compared to no heat exposure, daily exposure to extreme heat during the third trimester was associated with preterm birth (OR 1.61, 95% CI 1.55-1.67), as was exposure to nighttime extreme heat (OR 1.51, 95% CI 1.46-1.56), reported Shanshan Li, PhD, of the School of Public Health and Preventive Medicine at Monash University in Melbourne, Australia.

However, higher greenness mitigated these rates, as improved normalized difference vegetation index (NDVI) reduced daily extreme heat-associated preterm birth by 13.7% and nighttime extreme heat-associated preterm birth by 13%, while improved tree cover reduced both measures by 20.9% and 17.2%, respectively, they wrote in JAMA Pediatrics.

There were no associations between extreme heat in the first or second trimester and preterm birth.

“The results underscore the importance for healthcare providers to advise pregnant women to refrain from exposure to excessive heat, particularly during the third trimester,” Li told MedPage Today.

Li said the study sought to “provide evidence for the governments, communities, public health services, clinicians, and individuals to reduce risks of preterm birth related to heat exposure” by exploring the relationship between heat exposure during different trimesters and times of day with preterm birth.

They also looked at how exposure to greenness impacted those preterm birth rates, suggesting that higher greenness and improved tree cover could reduce heat-associated preterm birth.

“The findings also imply the necessity for governments and communities to prioritize the planting of additional trees and grasses,” Li said.

In an accompanying editorial, Heather Burris, MD, MPH, of the University of Pennsylvania’s Perelman School of Medicine in Philadelphia, and Allan Just, PhD, of Brown University’s School of Public Health in Providence, Rhode Island, noted that this study adds to the body of research that extreme heat fueled by climate change is “harmful for human health, including during pregnancy” and that identifying ways to intervene is vital. However, they wrote that “the magnitude of the association is higher than what is often observed in epidemiologic studies of other exposures with [preterm birth].”

They suggest that the study’s large effect estimate “may be a result of methodologic challenges of characterizing the relevant etiologic period inherent to [preterm birth] studies.” The editorialists suggested that the study could benefit from getting more granular and incorporating factors such as the role of humidity, sudden temperature changes, and heat waves.

Li and team noted some of these limitations, such as the lack of preterm birth subtypes in the dataset, the fact that measures of greenness did not capture intra-community variations, and that it’s difficult to account for an individual’s adaptation to their local climate.

Despite these suggestions, the editorialists said studies like these “are crucial to inform public health efforts to optimize health across the life span.”

Researchers analyzed data on pregnant women who lived in New South Wales during their pregnancy from 2000 to 2020. The dataset included information on pregnant people and infants. People without complete residential data and those who lived outside New South Wales during their pregnancy were excluded.

There were 1,225,722 singleton births (51.5% male, 48.5% female) delivered at a median 39 weeks’ gestation. Only 5.15% were born preterm (median age 35 weeks). Mean maternal age was 31, but the average for those who delivered preterm was slightly older. For daily extreme heat, preterm birth rates were 7.5% versus 4.9% for those unexposed, and 7.1% versus 4.9% for nighttime extreme heat, respectively.

Heat exposure data from the European Centre for Medium-Range Weather Forecasts Reanalysis, version 5, was broken down into average daily, weekly, and trimester temperatures for day and night. Extreme heat was above the 95th percentile of the location. Greenness was determined by NDVI from Moderate Resolution Imaging Spectroradiometer vegetation indices and tree cover pulled from satellite images.

Li said the team hopes to “expand this study on a global scale, delving into the economic implications associated with heat exposure and exploring strategies to mitigate this burden.”

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