Extreme heat exposure a risk factor for preterm birth

Pregnant women who have been exposed to extreme heat have higher chances of delivering their babies before term, while those residing in greener areas are less likely to experience the outcome, according to a study from Australia.

In a large birth cohort, exposure to daily extreme heat and nighttime extreme heat in the third trimester was associated with more than 50-percent greater odds of preterm birth compared with no heat exposure (odds ratio [OR], 1.61, 95 percent confidence interval [CI], 1.55–1.67 and OR, 1.51, 95 percent CI, 1.46–1.56, respectively). [JAMA Pediatr 2024;doi:10.1001/jamapediatrics.2024.0001]

Greenness, as measured using the normalized difference vegetation index (NDVI) and tree cover derived from satellite images, moderated the association between extreme heat exposure and preterm birth, such that pregnant women who resided in greener areas had a lower risk of heat-related preterm birth.

Building on the observed link between green spaces and reduced preterm birth risk, the study authors examined the potential preventative role of greenness and found that increasing greenness levels could prevent a substantial number of preterm births.

Results showed that improving tree cover to 30 percent could prevent up to 38.2 percent of heat-associated and 34.1 percent of nighttime heat-associated preterm births. Similarly, increasing the NDVI to the 75th percentile could prevent 21.8 percent and 19.8 percent of preterm births associated with exposure to daily extreme heat and nighttime extreme heat, respectively.

The analysis was based on data from 1,225,722 births (median age 39 weeks, 51.5 percent male), including 63,144 births that occurred before term (median age 35 weeks, 55.1 percent male). Daily extreme heat and nighttime extreme heat were defined as above the 95th percentile of community- and trimester-specific daily mean temperatures and nighttime temperatures, respectively.

The dangers of heat

According to the authors, exposure to high temperatures during pregnancy can lead to dehydration and increased body temperature, potentially leading to physiological changes. This is supported by animal data showing that elevated body temperature can lead to increased production of oxytocin and prostaglandins and other studies indicating that heat exposure may induce oxidative stress and inflammation—bodily processes that are associated with adverse pregnancy outcomes. [Theriogenology 2021;165:52-58; Am J Physiol Regul Integr Comp Physiol 2008;295:R611-R623; Environ Int 2013;61:77-87]

With regard to extreme heat at nighttime, the authors postulated that disrupted sleep and increased maternal discomfort during hot nights could contribute to adverse pregnancy outcomes. “Sleep disturbances increase preterm birth risk, and nighttime temperature extremes can disrupt sleep patterns, potentially leading to physiological consequences.” [J Womens Health 2022;31:755-757; Front Public Health 2022;10:946657]

The good news is that greenness can mitigate the dangers of heat exposure during pregnancy through a variety of health benefits, the authors said.

“For instance, increasing tree coverage in urban areas has been shown to reduce the urban heat island and lower ambient temperatures, leading to improved thermal comfort. Moreover, the presence of greenness in residential environments may encourage physical activity, which is associated with positive pregnancy outcomes. Greenness also contributes to reducing stress, improved air quality, and improved overall well-being, potentially buffering the impact of heat on pregnancy outcomes,” they pointed out. [Environ Res 2018;160:372-384; Curr Opin Pediatr 2019;31:237-243]

In a linked editorial, Drs Heather Burris and Allan Just, both from the University of Pennsylvania, Philadelphia, Pennsylvania, US, highlighted the clinical implications of the study’s findings. [JAMA Pediatr 2024;doi:10.1001/jamapediatrics.2024.0008]

“[I]t is becoming increasingly apparent that extreme heat is harmful for human health, including during pregnancy. Such data and studies are crucial to inform public health efforts to optimize health across the life span,” Burris and Allan wrote.