Experts Highlight The Dangers Of Extreme Heat On Pregnancy Outcomes
When summer temperatures soar, Nathaniel DeNicola, an ob-gyn in California specializing in environmental health, often sees an upsurge in pregnant women rushing to the hospital because they’re experiencing labor pains—sometimes well in advance of their due dates. Often, the contractions subside with rest and rehydration.
However, he says, studies show that the risk of these so-called “practice contractions” transitioning into true labor increases when the heat lasts more than a few days. Heat waves are “beyond an inconvenience,” he says, “they are a risk factor for a preterm birth.”
Yet, while pregnant patients are usually counseled to stay hydrated and cool, they might not understand why, exactly, those habits are so critical. “I don’t think the next step has been taken to say, ‘This isn’t just a matter of best practice for pregnancy. It’s not just a matter of helping you avoid some contractions. This is a risk factor for a pretty serious outcome,’” says Dr. DeNicola.
The connection between soaring temps and preterm birth has been formally studied now for nearly two decades thanks, in part, to Rupa Basu, an Oakland-based epidemiologist at the California Environmental Protection Agency. Back in the summer of 2007, Basu was pregnant with her second child when she started experiencing what felt like intense hot flashes. She drank lots of water, avoided the midday sun, and began combing through scientific literature to see if anyone had studied how pregnant women respond to high temperatures. If she was suffering in temperatures in the high 80s, what must it be like for women in areas of India and Africa where blistering heat and humidity are pushing the limits of human survival?
Basu discovered that no one had yet examined the question of whether high temperatures lead to premature births or other negative pregnancy outcomes. After her son was born, she collected state weather data and 60,000 records of summertime births across 16 California counties, paying special attention to babies born before 37 weeks, or full term. Sure enough, she found greater numbers of preterm births correlated with higher temperatures, and published her findings a few years later. “It was the first large-scale study looking at temperature and preterm delivery specifically, that found a positive association,” she says.
More and more data has since backed up Basu’s initial findings: Preterm birth rates jump one percent after four straight days of high heat, according to a study in the Journal of the American Medical Association that analyzed 53 million births from 1993 to 2017 in the nation’s 50 most populous cities. Similarly, in a 2020 literature review, every one-degree Celsius rise in environmental temperature brought a 5 percent increase in preterm birth. During a heat wave, it jumped to 16 percent.
Unfortunately, the planet is only going to get hotter. It’s no secret that the earth is warming—climate scientists predict that in the next four years, temperatures will rise between one and nearly two degrees Celsius higher than a 1850–1900 baseline. In the coming years, experts also predict that the frequency, intensity, season length, and duration of heat waves will become even more serious, which means these extreme weather events’ health impacts have “the potential to be so much worse,” Basu says, especially for pregnant women and their babies.
Raising The Heat Alarm
The early arrival of a baby before the 37th week of gestation is one of the leading causes of infant mortality, and is linked to a range of health problems for surviving infants, including lung disorders, anemia, heart defects, and stillbirth. Longer term, premature babies can experience trouble learning, vision and hearing problems, and behavior and mental health conditions.
Yet, even with more than a decade of research now clearly showing how vulnerable pregnant people are in extreme heat conditions, when the temperature warms, health officials often warn elderly people, outdoor workers, and pets about the risks. Pregnant people are too often overlooked, Basu says. “If people don’t realize that they’re at high risk, there won’t be any precautions taken,” she says.
That’s why health experts are starting to call for better public education campaigns and new legislation in the hopes of better protecting pregnant women and their babies in the face of worsening environmental conditions driven by climate change.
Pushing The Limits
How heat triggers preterm labor is still somewhat unclear. Doctors have found growing evidence that pregnant women exposed to extreme heat and dehydration may experience a reduction of blood flowing to the uterus and placenta, a boost in hormones, or an inflammatory response—which can in turn cause Braxton Hicks contractions and spur labor.
And when it comes to extreme heat, there seems to be a tipping point when rest and rehydration is no longer enough to stop the Braxton Hicks contractions, says DeNicola. “It converts into actual labor, where that hormone cascade continues and you start having organized contraction patterns that open the cervix. And once that starts, we really can’t stop it. We have ways to stall preterm labor, but we really can’t stop it.”
In the years since Basu first published her research, there’s been an explosion of studies examining how, exactly, increasing heat affects expectant mothers.
In February 2024, a study of 1.2 million births in Sydney, Australia found a strong association between exposure to extreme heat during the third trimester and an increased risk of preterm birth.The research also provided insight into how city planners might be able to help address the problem: The risk was lower for women living in areas with more green space, suggesting that tree cover and grassy areas incorporated into urban planning may be one way to help mitigate heat levels and improve health outcomes.
A growing number of studies also illustrate the wide health disparities for minority ethnic and racial groups in cities, including mothers who have premature or early-term births after a heat wave hits. In one recent study of 108 urban areas in the U.S., almost every city’s historically redlined neighborhoods (with racially segregated housing) were hotter than non-redlined neighborhoods due to a number of factors, including a lack of greenspace, trees, or bodies of water in the area. In some cases, the difference was nearly 13 degrees.
Scientists often consider factors such as higher education, income, and social status when studying health outcomes. And the research tends to show that the better off you are financially, the fewer problems you’ll have. That’s why University of Minnesota researchers investigating climate change and racial inequities in the Twin Cities were surprised when their study showed college educated Black women were more likely to experience premature labor and birth than white women without a college education.
Epidemiologist Morrison Luke Smith, one of the researchers involved in the study, says the finding likely has something to do with a phenomenon known as the “weathering effect.” Marginalized people suffer nearly constant stress from living with discrimination, which damages their bodies at the cellular level and can lead to poor health outcomes, he explains. “Heat is not a chronic stress,” Smith says. “It comes and goes—but it’s just one more thing.”
Breathing For Two
Heat isn’t the only health stressor pregnant women face in a warming world. Climate change sparks more frequent and intense wildfires, which have similarly been shown to affect expectant mothers and their babies. While it can sometimes be difficult to untangle the impact of poor air quality and heat, there are strong indications that exposure to contaminated air can lead to poor birth outcomes, says Camille Raynes-Greenow, an Australian epidemiologist at the University of Sydney studying how air contamination affects pregnant women in Bangladesh.
Exposure to wildfire smoke during pregnancy can increase the odds of preterm birth, particularly in the second and third trimesters, and can raise the risk of preeclampsia, a serious pregnancy and postpartum condition causing high blood pressure that can be life threatening.
Studies also show that air contaminants can cross the human placenta, directly exposing fetuses to that polluted air. This is especially concerning considering Dr. DeNicola says women breathe in more oxygen during pregnancy—they literally breathe faster during the first trimester due to increased estrogen and progesterone in the body. That could expose them to more fine airborne particles during a wildfire.
And last year’s fire season—where smoke drifted thousands of miles from Canada and painted New York City an eerie orange—showed that you don’t have to live in a wildfire-prone area to encounter “bad” air. Fine particle pollution from wildfire smoke is a nationwide health concern, according to Climate Central, a nonprofit formed by an independent group of scientists. Studies suggest that U.S. smoke exposure has increased 27-fold in the last decade, the group reports.
Work To Be Done
While there’s still a lot of research needed to unpack all the ways our health is impacted by heat and wildfires, experts are clear that pregnant women should know how both can increase the risk for preterm birth and poor birth outcomes.
Basu is sometimes tempted to paper grocery store bulletin boards warning women about environmental hazards, including heat waves and bad air quality, that may affect their pregnancies. She’s worried about what she’s learning from a recent California study examining how wildfire smoke in conjunction with heat waves affects pregnancies. “The combined effect is even greater than either one alone,” she says. Basu hopes that by showing strong evidence that heat and wildfires are causing adverse birth outcomes, health officials will work to promote better awareness for pregnant people, who are often overlooked in health advisory warnings. “All people who are pregnant are at increased risk,” she says.
And while Raynes-Greenow also doesn’t want to overly worry expectant mothers—especially since not everyone has the option to simply avoid the heat—she encourages them to take a sensible approach to keeping themselves comfortable.
Beating The Heat
Stay safe with these best practices.
Know The Signs
Signs of overheating include headache, dizziness, muscle cramps, nausea, and vomiting, according to the Centers for Disease Control and Prevention. If you’re experiencing any of these symptoms, it’s best to get in touch with your doctor. Similarly, on bad air-quality days, or during wildfire season, you can call your doctor to talk through your options for creating or moving to a healthy and safe environment.
Stay Up-To-Date
Dr. DeNicola says it’s important for pregnant people to be prepared for high heat and poor air-quality days. He recommends using mobile apps such as the government’s Heat Safety Tool and AirNow, which offer real-time temperature and air quality alerts, when planning outdoor activities.
Be Proactive
During a heat wave, try closing the curtains in your home during the hottest part of the day. Schedule outdoor activities in the early morning and late afternoon, and lay low during the midday heat. Wear loose, light-colored, breathable clothing. And if you don’t have air conditioning, sleep in the coolest part of the house, which is often on the ground floor. It’s also extremely important to stay hydrated—so grab your water or electrolyte beverages before you feel thirsty. “Always drink ahead of your thirst,” says Dr. DeNicola.
Breathe Better
If you live in a wildfire-prone area or in a big city experiencing bad air-quality days, it’s best to take measures to avoid breathing in fine particulate matter, particularly if you’re pregnant, according to the U.S. Environmental Protection Agency. The agency recommends using N95 respirator masks, which you can usually buy in hardware stores or drugstores, on days with particularly bad air quality. Other steps can include keeping doors with windows closed, adjusting your HVAC or air conditioning system to keep smoke from coming into the house, and adding a portable air filter or high-efficiency HVAC filter to your home.
Find Your Local Refuge
If you’re having a hard time finding somewhere cool to rest, you may be able to visit a community cooling center, or the waiting rooms of health facilities, libraries, and shopping malls. On bad air-quality days, you can also try going to these public centers, which may have better ventilation and air filtration systems.
link
