Heat exposure is linked with poor birth outcomes

Exposure to higher temperatures during pregnancy has been associated with adverse outcomes, including preterm birth and stillbirth, according to research published in T.he BMJ.

“Much more needs to be done to protect pregnant women from exposure to extreme heat, especially in the later stages of pregnancy,” Matthew Francis Chersich, PhD, of the Wits Reproductive Health and HIV Institute and the faculty of health sciences at the University of the Witwatersrand, Johannesburg, South Africa, he told Healio Primary Care. “In high-income groups this mostly affects air conditioning, but many other women cannot access the cooling spaces and some continue to do physical work until the end of the pregnancy.”

Chersich and colleagues conducted a systematic review and meta-analysis of heat exposure or heat adaptation studies and assessed pregnancy and delivery outcomes. They identified the studies through online searches on Medline and the Web of Science.

A total of 70 studies completed in 27 countries were included in the review. Among the countries, seven were considered to be low- or middle-income countries.

Forty studies have identified associations between higher temperatures and preterm delivery. In a meta-analysis of six studies, the researchers found that the odds of preterm birth increased by 1.16 (95% CI, 1.10-1.23) with heat waves and 1.05 (95% CI; 1.03-1.07) with each 1 ° C increase in temperature.

Chersich and colleagues found that all eight studies with information on stillbirths identified increases in stillbirths with higher temperatures. A meta-analysis of three studies showed that each 1 ° C increase in temperature was associated with an increase of 1.05 (95% CI; 1.01-1.08) in stillbirths.

The researchers also stated that 18 studies found that higher temperatures were associated with lower birth weight. They noted that while most studies reported slight changes in birth weight, even small changes would have a major impact on public health as exposure to high temperatures is common and increasing.

Associations between higher temperatures and birth outcomes appeared to be more pronounced among women living in low- and middle-income countries, suggesting they are particularly at risk, according to Chersich and colleagues.

Chersich said an “imbalance between heat production and heat loss” could result in the association between heat exposure and poor birth outcomes.

“The fetus and placenta generate a great deal of heat due to their rapid metabolism,” he said. “Hours of exertion during labor similarly generate large amounts of heat that can prolong labor, perhaps explaining some of the major stillbirths in settings where caesarean section is not readily available.”

She added that doctors generally don’t advise pregnant women to protect themselves from extreme heat, “but that should change.”

“Exercising outdoors during hot spells and adequate hydration and access to cooling areas would be helpful tips, especially as the world warms up further,” he said.