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Heat exposure during pregnancy was associated with a higher risk of severe maternal morbidity, a large retrospective cohort study suggested.

Among over 400,000 patients at a Southern California health system, high exposure -- defined as ≥80th percentile of the proportions -- to extreme heat days during pregnancy and the third trimester specifically were associated with a 27% and 28% increased risk of severe maternal morbidity, respectively (P<0.001 for both), reported Jun Wu, PhD, of the University of California Irvine, and colleagues.

The magnitude of associations mostly increased from the least severe (a daily maximum temperature >75th percentile lasting for 2 days; OR 1.32, 95% CI 1.17-1.48, P<0.001) to the most severe heatwave exposure (a daily maximum temperature >95th percentile lasting for 4 days; OR 2.39, 95% CI 1.62-3.54, P<0.001), they noted in .

The findings during the third trimester suggest that this may be a "critical exposure window" for mothers-to-be, co-author Anqi Jiao, a PhD candidate at the University of California Irvine, told 番茄社区app.

Greater associations were noted in women whose pregnancies started in the cold months of November through April (OR 1.37, 95% CI 1.24-1.53, P<0.001), suggesting that mothers who started their pregnancy in the cold season experienced more heat days during their final trimester, while mothers who started their pregnancy in the warm season spent their third trimester in winter or spring.

Severe maternal morbidity is considered "a near-miss for maternal mortality," the authors noted, with the 21 indicators of severe maternal morbidity at delivery, such as sepsis and acute myocardial infarction.

"Despite improvements in prenatal care coverage and quality due to technological advances (e.g., improved screening and treatment for medical conditions during pregnancy and better identification and interventions for risk factors associated with adverse pregnancy outcomes), the prevalence of severe maternal morbidity has continued to increase in the U.S.," they wrote.

The authors also found that greater associations were seen among women with lower educational attainment (OR for high exposure to extreme heat days during pregnancy 1.43, 95% CI 1.26-1.63, P<0.001), which they considered as a proxy for socioeconomic status.

"Our observation of worse health outcomes among women with lower socioeconomic status may reflect the broader impacts of the persistent and pervasive social injustice issues, including higher adverse exposure levels, more cumulative stressors, more underlying health conditions, and lack of resources and opportunities among these vulnerable populations," Wu and colleagues wrote.

Jiao pointed out that anyone who spends more time indoors, such as in their workplace, would have a different temperature in that environment, "which means even though the temperature outside is really high ... they may not be exposed to the extreme heat."

The main take-away from this study is that women must follow "mitigation measures to reduce their extreme heat exposure during pregnancy," she added. For clinicians, she stressed the importance of educating patients about heat exposure.

"You cannot change your age. You cannot change your race and ethnicity, and also for your income or education, it's a little bit hard to change," Jiao said, citing common risk factors for morbidity during pregnancy. One strength of this study is that it identifies a preventable risk factor for severe maternal morbidity, she noted.

The authors said that having access to green space appeared to reduce the impact of heat by providing shade, and provided opportunities for socialization and exercise. Other mitigation measures that Jiao recommended included using air conditioning and fans, and "self-dousing" with water.

For this population-based epidemiological cohort study, Wu and team included 403,602 singleton pregnancies at Kaiser Permanente Southern California from January 2008 through December 2018. Mean age of women was 30.3 years, 50.8% were Hispanic, 26.2% were white, and 12.6% were Asian.

Of these women, 0.9% experienced severe maternal morbidity during delivery hospitalization, measured by 20 subconditions, per the CDC's list, excluding blood transfusion.

Moderate, high, and extreme heat days were defined as daily maximum temperatures exceeding the 75th, 90th, and 95th percentiles of the time series data from May to September 2007 to 2018 in Southern California, respectively. The researchers assigned daily maximum temperature values during pregnancy to each individual based on their geocoded home addresses accounting for residential mobility.

One limitation to the study was the use of ambient temperature as a proxy for heat exposure at the individual level, Jiao said, adding that future studies could improve upon this measurement by using the surface temperature of buildings and roads. Additionally, the study population was from a single healthcare organization, limiting generalizability.

Jiao said that she and her team next plan to study the impact of air pollution from wildfires on pregnancy outcomes.

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