Exposure to extreme heat during pregnancy has shown to be associated with a higher risk of severe maternal health problems, according to a new study.
The study published Thursday in the JAMA Network Open found a significant association between long-term heat exposure during the third trimester and severe maternal morbidity (SMM) — defined as severe and unexpected conditions during labour and delivery. The risk of SMM in these circumstances showed an increase of 27 per cent.
“Associations observed in the third trimester indicated that late pregnancy might be a more sensitive window to environmental heat,” the study says.
Risks were still “significantly associated” in short-term exposure as well under “all heatwave definitions”: moderate, high and extreme.
The rate of SMM in 2014 was already three times more than 20 years prior, the study says.
Greater risks were observed among pregnant individuals with a “lower educational attainment,” the study notes, or whose pregnancy started in the cold season between November and April. Age played a role as well, with an increased risk presented among pregnant people under the age of 25 or older than 35.
“Adolescent mothers are more likely to be in unfavourable social or physical conditions and may have limited knowledge or awareness to protect themselves against extreme heat,” the study said. “Pregnancy complications and risk factors can be more common among older mothers, possibly due to the physiological changes from aging, which may exacerbate heat effects.”
403,602 pregnancies were studied from 2008 to 2018 in Southern California. Researchers conducted a retrospective population-based epidemiological cohort study, where preexisting data is used to examine the relationship between an exposure and an outcome.
About one-third of SMM indicators are associated with cardiovascular, cerebrovascular or hypertensive conditions, the study says, adding that the cardiovascular subconditions of SMM may play a critical role in associations between heat exposure and SMM.
Scientific studies in recent years have also shown that cardiovascular conditions have become a leading cause of pregnancy-related deaths.
Hot weather in general tends to put a strain on the human heart, as the body has to work extra hard to keep its core temperatures at normal levels, according to the British Heart Foundation.
One heat mitigation strategy that the study notes is green space.
“Besides reducing heat by providing shade from trees, studies have shown that more social support and physical activity induced by residential green space can lead to better health conditions,” it says.
The study’s findings come following a summer with record-breaking global temperatures. Earth had its hottest three-month period on record this year, according to the European Union-funded Copernicus Climate Change Service (CCCS). It was the hottest August on record, and the second hottest month ever after July 2023.
Rising temperatures have long been met with alarm bells by climate experts. Warnings became even more urgent with every worsening heatwave this summer.
“What we are observing — not only new extremes but the persistence of these record-breaking conditions, and the impacts these have on both people and planet — are a clear consequence of the warming of the climate system,” says Carlo Buontempo, director of CCCS on the World Meteorological Organization (WMO) website.