Extreme heat exposure associated with a higher risk of severe maternal morbidity for pregnant women: Study

Profile of a pregnant woman in a one-piece bathing suit with a bright, sunny beach background
What new research says about extreme heat exposure and health risks for pregnant women. (Getty Images)

As many of us learned over this sweltering summer, extreme heat can pose serious health risks, including dehydration, heat exhaustion and heatstroke. According to a just-released study, pregnant women have another cause for concern: an increased risk for severe maternal morbidity (SMM). As climate scientists warn that next summer could be even hotter, experts break down the findings and tell us what expectant mothers should know about staying safe in extreme heat.

What is severe maternal morbidity?

The World Health Organization defines maternal morbidity as "any health condition attributed to and/or aggravated by pregnancy and childbirth that has negative outcomes to the woman's well-being." These outcomes could be short- or long-term, with the Centers for Disease Prevention and Control citing 21 indicators, or subconditions, including eclampsia, blood transfusion, hysterectomy, ventilation, cardiac arrest, sepsis and shock.

While research published this June showed an overall decline in maternal mortality rates from 2008 to 2021 among women delivering in U.S. hospitals, the prevalence of SMM increased during that same time period. SMM rates were also higher for patients of a racial or ethnic minority group, while advanced maternal age, racial or ethnic minority group status, comorbidities (such as hypertension or diabetes) and C-sections were associated with an increased risk of maternal mortality and SMM.

What the study says

Published by the Journal of the American Medical Association (JAMA) on Thursday, the study found that long- and short-term extreme heat exposure during pregnancy was associated with a higher risk of SMM. Researchers noted that "these results might have important implications for SMM prevention, particularly in a changing climate."

What are the key findings?

Using data from Kaiser Permanente Southern California as a cohort study, researchers looked at 403,602 pregnancies — 3,446 of which involved SMM — taking place between 2008 to 2018. To determine long-term heat exposure, researchers measured the proportion of different heat days — moderate, high or extreme heat — during each pregnancy and by trimester. Short-term exposures, meanwhile, were calculated using heat waves of varying durations during the last week of pregnancy.

A caveat: While researchers were able to estimate green-space exposure using street-view images of the area surrounding residential addresses at the time of delivery, they did not have available data to factor in individual-level behavior, such as spending time indoors, cooling off in the shower or cranking up the air conditioning. And because the study was limited to pregnancies that occurred in Southern California, its authors recognize the need for "further studies in other regions with various climates and diverse populations."

The findings:

  • Long-term heat exposure during pregnancy — defined as high exposure to extreme heat days — was associated with a 27% increase in risk of SMM during pregnancy and a 28% increase during the third trimester.

  • Short-term heat-wave exposure was also "significantly associated" with elevated SMM risks.

  • There was a significantly higher association between heat exposure and SMM among mothers who did not attend college.

  • Mothers whose pregnancies began in the cold season (and were therefore further along into their pregnancies during periods of extreme heat) also saw higher associations between heat exposure and SMM.

"In our analysis, we observed associations between extreme heat exposure and adverse cardiovascular, cerebrovascular or hypertensive conditions in SMM (including aneurysm, acute myocardial infarction, cardiac arrest/ventricular fibrillation, conversion of cardiac rhythm, eclampsia, heart failure/arrest during surgery or procedure, pulmonary edema/acute heart failure, puerperal cerebrovascular disorders and shock)," study author Anqi Jiao of the Department of Environmental and Occupational Health at the University of California, Irvine, tells Yahoo Life. "It suggests that those subconditions of SMM may play a critical role in associations between heat exposure and SMM."

Adds Jiao: "We observed a higher vulnerability among mothers with lower educational attainment. This observation of worse health outcomes among women with lower socioeconomic status may reflect the broader impacts of the persistent and pervasive social injustice issues."

What experts say

Dr. Alison Cowan is an ob-gyn and the head of medical affairs at Mirvie, which develops prediction tools for life-threatening pregnancy complications like eclampsia. She tells Yahoo Life that the study "adds further evidence" of the risks extreme heat can pose to pregnant women.

"We've known before that heat is associated with adverse pregnancy outcomes," she says. "It has been associated with having low-birthweight infants and even with stillbirth in some studies. So we've known that there are some risks associated with heat exposure, and I think the current study adds to that body of evidence showing us that heat exposure also increases the risk of having a severe maternal morbidity."

"Extreme heat also affects other aspects of pregnant people’s health and well-being," says Dr. Mary T. Jacobson, ob-gyn and chief medical officer for the women's health resource Hello Alpha. One example of note: "Heat waves are associated with increases in intimate partner violence, which is already a leading cause of death for pregnant people in the U.S." What's more, extreme heat exposure has also been associated with an increased risk of preterm birth and, as Jiao's research team has previously reported, premature rupture of membranes.

Cowan was particularly drawn to the study's emphasis on worsening cardiovascular outcomes in pregnancy, noting that heat exposure has also been associated with adverse cardiovascular outcomes in the nonpregnant population. That's significant given the prevalence of cardiovascular-related complications — like high blood pressure disorders such as preeclampsia — in cases of SMM and maternal mortality.

Why it matters

As Jiao and her colleagues note, the study's findings have "important implications" in light of our "changing climate."

"Climate change will continue to impact all facets of health with increasing severity and duration of extreme heat events ... ," Jiao says. "Given the susceptibility of women during the pregnancy period, extreme heat would continue to pose threats and increase the risk of different adverse pregnancy outcomes including but not limited to SMM."

"Climate change is considered an urgent women's health issue because of its known associations with these adverse outcomes," adds Cowan, pointing to the American College of Obstetricians and Gynecologists' official position on the topic. There should be a level of urgency with climate change research, she says, which will hopefully pave the way for initiatives that combat its effects. Jacobson adds that this is an opportunity for "health care policymakers and community organizers" to "design and implement targeted interventions" to help pregnant women.

What can pregnant women do?

First things first: Don't panic.

"It's important for pregnant women not to feel afraid," says Cowan, who is optimistic that the warnings about SMM and maternal mortality rates have led to "a lot of systematic changes" and improved outcomes for patients. "And it's not that they have to lock themselves indoors in full air conditioning at all times, but it's really about common sense."

That means taking measures to reduce one's exposure and health risk. "Limiting time in high-risk heat exposure and staying hydrated when you are going to be exposed to heat is really important. Make sure that you have a plan for drinking enough water if you're going to be out and about during the day, but you can still live your life normally.

"It's really about making sure that if you're feeling the heat — if you're starting to feel things any lightheadedness, if you feel extremely hot, if you're feeling like your heart is racing — those are all signs that it's time to get indoors and just take a break," she continues. "So, just listening to your body and those commonsense things, I think, is the most important thing that most people can do."