Why extreme heat is dangerous for pregnancies and how to protect yourself

Persistent high temperatures during the summer likely contribute to more premature births, especially among socioeconomically disadvantaged groups, according to a recent study from a group of scientists in six U.S. states.
The stakes are high, scientists say, as preterm deliveries are a leading cause of infant mortality, according to the Centers for Disease Control and Prevention. And heat waves are only increasing in frequency, duration and intensity, according to the Environmental Protection Agency.
“Our current environment and climate issues may have an impact not only today for our pregnant individuals and their infants that are born, but potentially into the future,” said Dr. David Nelson, division chief of maternal-fetal medicine at UT Southwestern Medical Center and medical director of maternal-fetal medicine at Parkland Health.
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In North Texas, temperatures have reached triple digits for days on end this month and excessive heat warnings have been issued. Last week, Dallas County confirmed the first heat-related death of the summer.
And while this season is proving less hot than last year so far, the past week has seen year-record highs, a concern for pregnant people and their children.
Research published in the Journal of the American Medical Association in May found heat waves in U.S. cities were associated with earlier births.
Premature births happen before 37 weeks of pregnancy are completed, and early-term babies are delivered at 37 to 38 weeks of pregnancy. In 2023, about 10.4% of all births were preterm and 29.8% of babies were born early-term in the U.S., according to CDC data. For Black women, about 14.7% of births were preterm.
The study published in May found that among its sample, the daily rate of preterm births increased by around 2% after four days in a row of higher temperatures, while the daily rate of early-term births increased by 1%.
Based on the average number of preterm and early-term births occurring daily in Texas, this would be around three additional early-term births and two additional preterm births on days in a heat wave, according to lead study author Lyndsey Darrow, a professor of epidemiology at the University of Nevada.
That’s of the roughly 1,062 births that occur daily in the state, which include around 117 born preterm and 319 born early, according to CDC data and Darrow’s calculations.
“These effect sizes are very small,” Darrow told The Dallas Morning News. “But … given the vast majority probably are avoiding the heat, the fact that we still see an overall tiny population level response suggests that there are subgroups of people that are experiencing much, much stronger effects … and those are probably the people being actually exposed to the hot temperatures.”
The study analyzed 53 million births in the 50 most populous metro areas, covering roughly half of U.S. births over 25 years, from 1993 to 2017. The data came from the National Center for Health Statistics, and Dr. Nelson said the study is unique in the large data set it examined and the scope of its analysis.
Four Texas metro areas were analyzed in the study, including urban and suburban areas in Dallas, Houston, San Antonio and Austin.
The study did not have data from rural areas. Because populations in those areas are more spread out, it’s harder to determine whether all patients experienced the same heat, Darrow said.
The study looked at births in the warm season, roughly from May to September, over a 25-year period. A heat wave was defined as four or more days in a row over the 97.5th percentile for temperature in a city, relative to normal local temperatures — 101.5 degrees Fahrenheit for the daily max temperature in the Dallas-Fort Worth metro area, Darrow said.
Last summer was the world’s hottest on record, according to NASA scientists, and was Texas’ second-hottest summer on record behind 2011. In the summer of 2023, the Dallas-Fort Worth area endured 47 days with triple-digit temperatures. Scientists say the heat is only getting worse with climate change and global warming.
“The climate crisis has a very real impact on our bodies and our reproductive health,” said Adelle Monteblanco, an assistant professor of public health at Pacific University in Oregon. “Climate change takes a toll on our bodies.” Monteblanco was not involved in the study.
Darrow’s study names several possible reasons that hotter temperatures could contribute to earlier births and more negative health outcomes. Heat stress and dehydration reduce blood flow to the uterus and placenta, which could affect hormone levels that induce labor or the ability of the uterus to contract.
Heat stress may also trigger an inflammatory response that could affect the induction of labor, or extreme heat might cause a pregnant person’s water to break early, according to the study. If heat worsens a pregnant person’s existing diseases or causes fetus distress, it may encourage doctors to consider delivering the baby early, despite potential risks.
The study did not research the science behind preterm birth. These are theories as to how heat contributes to earlier births, and aren’t proven, said Dr. Nelson of UT Southwestern and Parkland Health. Dr. Nelson was not involved in the study.
“We don’t quite understand all of the different causes or reasons for preterm birth,” Dr. Nelson said. “This is one of those puzzle pieces that we’re trying to understand how it fits into the bigger picture.”
The people most affected by the study’s findings, Darrow said, are likely those who have a hard time avoiding the heat while pregnant. For people working jobs with more outdoor exposure, who take public transportation or who can’t afford consistent air conditioning, heat waves might pose a greater health risk.
Yashica Halton, a 28-year-old from Dallas, recalls the heat last summer as “tormenting.” In July 2023, Halton was homeless and learned she was two months pregnant. She lived in Longview and stayed at a women’s shelter that she said didn’t always have working air conditioning.
“The heat here … is terrifying,” Halton said. “You’re going to see people going through so much trauma and anger over heat alone. As a homeless person, you can imagine, and being pregnant — it was hard.”
Halton said she would faint and vomit easily during her pregnancy, and because she was severely anemic, she took iron pills and adjusted her diet.
She said she had to spend a lot of time outdoors in triple-digit temperatures and would use her food stamps to buy bottled water. She eventually got a job at a Chick-fil-A, but didn’t have a car. In the August heat, she would walk to the bus stop, wait nearly a half hour sometimes for it to arrive and then hope that it had air conditioning for her ride to work.
After a couple weeks, she quit.
“It was too hard on my body with the effects of the pregnancy,” Halton said.
Halton was one of thousands of women experiencing homelessness while pregnant in the U.S.. A study from the University of Southern California found the prevalence of unhoused pregnant patients increased by roughly 72% from 2016 to 2020. For many without a home, the heat is hard to escape.
Darrow’s study found pregnant people who were 29 years of age or younger, had a high school education or less, and belonged to a racial or ethnic minority group, experienced even higher rates of preterm and early-term birth — 4% and 3%, respectively, compared to 2% and 1% in the general sample — following consecutive days of high temperatures.
“Not everyone lives near an OB/GYN or has transportation to a clinic,” Monteblanco said. “Not everyone has a smartphone or the internet. Not everyone can control how often they drink cold water, or when they leave for the bus, or how many breaks they get at work.”
Pregnant people in Texas face extra challenges to obtaining care. Texas ranks second to last in the nation for women’s health care, according to the Commonwealth Fund.
Texas also has the highest rate of uninsured people in the country, with around 16.6% of the population uninsured, according to 2022 U.S. Census data. That’s nearly 5 million Texans without health insurance.
Preterm birth puts an economic burden on health care systems. The total cost of preterm birth for the U.S. in 2016 was estimated to be around $25 billion, or roughly $64,800 per preterm birth, according to a 2021 study.
“Medical care services for children born preterm comprise probably the largest category of societal costs that we see in our space for obstetrics and gynecology,” Dr. Nelson said. “It is an incredible burden on the health care system.”
Local organizations, agencies and clinics seek to ease the burden of summer’s hottest days on people vulnerable to the heat in Dallas.
Dallas County Health and Human Services offers services to protect low-income residents in the county from heat through the Weatherization Assistance Program. Residents can apply for home repairs and installations to help reduce their energy usage and utility costs.
The City of Dallas encourages residents to use any of the city’s public facilities for temporary relief from the heat during business hours, such as the Dallas Public Library or the city’s recreation centers.
Other organizations are also stepping in to help. The Salvation Army operates cooling centers throughout North Texas. Cornerstone Baptist Church is offering free air conditioning units to eligible applicants. OurCalling, an outreach center near downtown Dallas, offers resources to protect from heat and operates a women’s center for women experiencing homelessness.
For support after birth, Parkland Health’s extending Maternal Care After Pregnancy program, or eMCAP, reaches women living in underserved areas of Dallas County. The program, in partnership with UT Southwestern Medical Center, offers postpartum follow-up care for 12 months after delivery and seeks to address health care disparities.
In Halton’s case, she gave birth in March, full-term, despite her health concerns and heat exposure. Her daughter wasn’t breathing after the delivery and had to be taken to a neonatal intensive care unit, but both Halton and her baby were healthy following the pregnancy. She is grateful for making it through that tormenting summer.
“That was only the grace of God,” Halton said.