Heat exposure linked to poor birth outcomes

Exposure to higher temperatures during pregnancy was associated with adverse outcomes, including preterm birth and stillbirth, according to research published in The BMJ.

“Much more needs to be done to protect pregnant women from exposure to extreme heat, especially late in 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, told Healio Primary Care. “In high-income groups this mostly involves air conditioning, but many other women can’t access cooling spaces and some continue doing physical work until late in pregnancy.”

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

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

Forty studies identified associations between higher temperature and preterm birth. 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 heatwaves and by 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 had identified increases in stillbirths with higher temperatures. A meta-analysis of three studies showed that each 1°C increase in temperature was associated with a 1.05 (95% CI; 1.01-1.08) increase in stillbirths.

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

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

Chersich said that an “imbalance between heat production and heat loss,” may be driving the association between heat exposure and poor birth outcomes.

“The fetus and placenta generate a large amount of heat given their rapid metabolism,” he said. “The hours of exertion during labor similarly generates large amounts of heat which may prolong labor, perhaps explaining some of the increased stillbirths in settings where caesarian section is not readily available.”

He added that physicians do not typically advise pregnant women to protect themselves against extreme heat, “but that should change.”

“Outdoor exertion during hot periods and adequate hydration and accessing cooling areas would be useful advice, especially as the world heats further,” he said.