A first-of-its-kind study led by the University of California, Irvine found that exposure to extreme heat had an association with severe maternal morbidity (SMM). The findings add to existing evidence of the impact of extreme heat on other adverse pregnancy outcomes, such as preterm birth, premature rupture of membranes, low birth weight, and stillbirth.
Corresponding author Jun Wu, PhD, professor of environmental and occupational health at the UCI Program in Public Health, collaborated with researchers at Kaiser Permanente and other institutions to conduct the study.
Their findings are published in JAMA Network Open.
From 2008 to 2018, a total of 403,206 pregnancies from Kaiser Permanente Southern California were analyzed, along with maternal characteristics, including socio-demographics, self-reported lifestyle, and medical history. By performing the time-to-event analysis, Wu and team were able to examine associations between long- and short-term heat exposure and SMM.The findings showed that both long- and short-term maternal exposure to environmental heat during pregnancy was significantly associated with higher risk of SMM that occurs during delivery hospitalization.
Despite improvements in prenatal care coverage and quality, the prevalence of SMM has continued to increase in the U.S. SMM is considered a near-miss for maternal mortality. The Centers for Disease Control and Prevention define SMM as severe and unexpected conditions during labor and delivery, such as amniotic fluid embolism, acute myocardial infarction, acute kidney failure, cardiac arrest, disseminated intravascular coagulation, heart failure, severe anesthesia complications, and sepsis.
“It is imperative to identify preventable risk factors for SMM to build targeted interventions, “ said Wu. “Exposure to extreme heat is one of the risk factors for SMM that has been exacerbated by our changing climate.”
For the past few decades, extreme heat episodes have become more and more frequent. The National Oceanic and Atmospheric Administration tracked July 2023 as the hottest on record ever. The severity and duration of extreme heat events will continue to increase and pose more threats to pregnant women due to their susceptibility.
We discovered that mothers with lower educational attainment were found to be more susceptible to heat-related SMM risks, probably due to limited knowledge and awareness that they have to prevent themselves from extreme heat during pregnancy.”
– Jun Wu, PhD
“Women who started their pregnancy in the cold season from November through April might be more vulnerable, as they would experience the hottest period in Southern California (May-September) in their late pregnancy,” said Wu. “We discovered that mothers with lower educational attainment were found to be more susceptible to heat-related SMM risks, probably due to limited knowledge and awareness that they have to prevent themselves from extreme heat during pregnancy.”
Importantly, the study also found associations between heat exposure and cardiovascular events during labor and pregnancy. Extreme heat may increase the risk of cardiovascular-related outcomes by leading to volume depletion and electrolyte imbalance, inducing inflammation and hypercoagulable states, and increasing heart rate and metabolic state. Existing literature has reported such associations in the general population, while the evidence of associations in pregnant women is lacking. Cardiovascular conditions are now a leading cause of maternal deaths in the U.S.
“Our results suggest that the cardiovascular sub-conditions of SMM may play a critical role in the associations between heat exposure and SMM, which might have important implications for SMM prevention in the changing climate,” said Anqi Jiao, the first author of the study and a current doctoral candidate at UCI Public Health.
Additional authors include Darios Getahun, MD, PhD, Chantal Avila, MA, Vicki Chiu, MS, Jeff Slezak, MS, and David A. Sacks, MD, all from the Kaiser Permanente Southern California Department of Research & Evaluation; John Molitor, PhD, from Oregon State University; Tarik Benmarhnia, PhD, from University of California, San Diego; Jiu-Chiuan Chen, MD, ScD, from University of Southern California; and Yi Sun, PhD, from University of California, Irvine Program in Public Health. Dr. Getahun is also affiliated with the Kaiser Permanente Bernard J. Tyson School of Medicine.
