Will Climate Change Lead to Worsening Birth Outcomes?

Exposure to extreme heat during pregnancy may be tied to higher risks of preterm delivery and stillbirth, according to a systematic review and meta-analysis.

Pregnant people exposed to higher temperatures were more likely to have a preterm birth, with the odds rising 5% with every 1°C increase (95% CI 3%-7%) and 16% during heatwaves (95% CI 10%-23%), reported Matthew Francis Chersich, PhD, of the University of the Witwatersrand in Johannesburg, South Africa, and colleagues.

Risk of stillbirth increased 5% per every 1°C (95% CI 1%-8%), the researchers wrote in The BMJ.

The group also observed associations between rising temperatures and low birth weight; however, the relationship was not as strong or consistent compared to the outcomes of preterm birth and stillbirth. Effects were strongest in women who were of lower socioeconomic status.

“Pregnant women merit a place alongside the groups typically considered as at high risk for heat related conditions,” Chersich and colleagues wrote. As the frequency and intensity of heatwaves are increasing, the group stated that more research and policy interventions should be a “high priority.”

“Right now, we only talk about vulnerable populations for heat as children or the elderly, but very few people talk about pregnant women,” said Shao Lin, PhD, an epidemiologist from the University at Albany in New York, who studies the impact of climate change on human health. Lin, who was not involved in this study, noted that pregnant women experience physiological changes that affect their metabolism and circulation, which may make them more susceptible to heat stroke.

While the effect sizes in this study were small, Lin said, pregnant women may face significant risks as the incremental temperature increases across the globe get larger.

In this review, Chersich and colleagues looked for clinical studies investigating the association between heat exposure during pregnancy and preterm birth, birth weight and stillbirth.

The review included 70 studies in total, the majority of which investigated the impact of heat on preterm birth. The studies were set in 27 countries, seven of which were low or middle income. Two-thirds of the studies were published after 2015, and all but two since 2000.

There were 40 of 47 studies that reported associations between rising temperatures and preterm birth, 18 of 28 reporting associations on low birth weight, and eight studies which all reported associations on stillbirth.

The median rate of preterm birth in all included studies was 5.6%. Eleven studies reported that preterm birth was highest in women younger than 25 or older than 35, and six studies reported that there was a higher rate of preterm birth among women in lower socioeconomic groups. In four studies from the U.S., researchers found that Black women had a higher incidence of preterm birth than white women.

A majority of the studies covering preterm birth reported dose-response associations, where rates of preterm birth rose with increasing temperatures or longer heat exposures.

There was a median of 6.2 per 1,000 stillbirths in all the studies included in this review. The meta-analysis reported that there was a 24% increased risk of stillbirth at lags measured on individual days during the last week of pregnancy (OR 1.24, 95% CI 1.12 to 1.36) and more than three times the risk when effects were measured across one whole trimester or the entire pregnancy (OR 3.39, 95% CI 2.33-4.96).

For the outcome of low birth weight, the median rate was 3%. The median observed risk of low birth weight associated with high temperatures was 1.09 (interquartile range 1.04-1.47). But researchers noted that the impacts of temperature on weight were generally small.

Chersich and colleagues wrote that their review highlights a need to research interventions that can reduce heat-related issues in pregnant women. This includes providing home-based services from community health workers, giving financial support to pregnant people during the later stages, monitoring maternal hydration, and cooling labor wards in low and middle income countries.

The group recognized that this research was limited by different temperature standards in the studies, as well as exposures like air pollution that may have impacted birth outcomes. Additionally, around a third of the included articles did not have detailed descriptions on study population and setting, and a quarter inappropriately used analytical methods.