Rising temperatures raise risk of stillbirths and preterm births in the U.S.

Climate change has added at least a week of dangerously hot days for pregnant women in every state in the country, increasing the risk of stillbirths and premature births, according to a , a nonprofit research organization.

Those extreme heat days also endanger the , through high blood pressure, diabetes and other ailments.

The increases, which were observed over the last five years, have been most dramatic in the Southwest, Climate Central’s analysis found. California, Colorado and Nevada experienced an additional 34 risky days. Texas and New Mexico are right behind with 33 days.

Worldwide, global warming at least doubled the number of dangerously hot days for pregnant women in 90% of the world’s countries between 2020 and 2024, according to the study. In the United States it almost doubled the number of days. 

“It's one thing to talk about polar bears, but it's another thing to to find evidence that the woman down the street or your sister who's pregnant has increased risk due to these really common exposures, heat being one of them,” said Dr. Bruce Bekkar, a women's health physician and an authority on climate change’s dangers to human health.

What Climate Central is calling “pregnancy heat-risk days” are reached when the maximum temperatures are higher than 95% of historic local temperatures, a threshold described as associated with increased risks of premature births. 

The group says its analysis is the first to quantify how climate change is increasing dangerously hot days for pregnant people across the world.

“It's something that women in the United States are experiencing, it’s something that women all over the world are experiencing,” said Kristina Dahl, the vice president for science at Climate Central. “If you are pregnant, being aware that exposure to extreme heat increases your chances of preterm birth, stillbirth, gestational diabetes, a number of different health risks is important, so that you can get yourself to a cool place when it's hot, and make sure that you're hydrated.”

Bekkar said extreme heat was pushing more pregnancies into high-risk territory, especially in places already struggling with limited health care access. The risks are especially acute for Black and Hispanic women. 

“One of the huge benefits I think of this report is it allows us for the first time to actually see how many days increase there are over the last five years,” he said.

In 2020, Bekkar and others published of the cumulative findings of 57 studies that tied heat or air pollution to birth outcomes in the United States. Bekkar said he was spurred to examine the data after a study from the noted that the vast majority of the effects of climate change could fall on children under the age of five.

“What we found hiding in plain sight was all this evidence that both heat and air pollution were already strongly associated with preterm birth, low birth weight and stillbirth across the continental U.S. -- that it was no longer a theoretical risk,” he said. 

Four studies linked high temperatures to an increase of between 8.6% to 21% in the chance of a premature birth. Another found that between May and September, for every temperature increase of 1 degree Celsius in the week before delivery, there was a 6 percent greater likelihood of a stillbirth.

The wide variety of research approaches prevented the researchers from doing a meta-analysis of the data from the different studies, Bekkar said. 

A worsening lack of adequate care adds to the danger for some pregnant women, whether because of unaffordable health insurance or , pockets where people have to drive an hour or more to get emergency treatment, Bekkar said. 

“They usually don’t have a voice,” he said. “I think there are a whole lot of people suffering even more than we know because we’re not hearing from them.” 

Other factors aside from temperature and prenatal care that help to determine risk: whether someone has access to air conditioning, has the resources to run air conditioning or even the availability of drinking water. 

Beyond staying hydrated and seeking a cool or air-conditioned place, pregnant women should consult with their doctors when temperatures begin to rise.

"It's something that patients can bring to their doctors and say, 'Hey, heading into summer, what do you recommend to keep me healthy and safe during this hot season?'" Dahl said.

Medical advances means more premature babies survive in the United States than previously but some are left with permanent health problems and disabilities. A 2022 study from the found such health and developmental problems among some babies born between 22 and 26 weeks as cerebral palsy, vision and hearing loss and a need for braces, walkers or wheelchairs.

To determine how climate change affected the frequency of pregnancy heat-risk days, Climate Central used its , which models climate change's influence on daily temperatures. Researchers modeled a scenario without human-caused climate change and then compared how each state would have experienced pregnancy heat risk days with and without climate change.

All states experienced more pregnancy heat-risk days than they would have in the scenario modeled without climate change. In fact, every state would have experienced at least 29% fewer of these days than it did, if climate change were not a factor.

Across the world, climate change added at least a month’s worth of pregnancy heat-risk days in nearly one-third of countries, according to the study. For some countries all of the pregnancy heat risk days were caused by climate change, though that was not true in the United States. The state that came the closest was Hawaii, where 20 of the 21 days were attributed to climate change.

The most number of pregnancy heat-risk days were added in developing countries, including in the Caribbean, Central and South America, the Pacific Islands, Southeast Asia, and sub-Saharan Africa. They also contributed the least to global warming, Climate Change pointed out.

And as Dahl noted, “It's not just the absolute temperature that you're exposed to, but it's the temperature that's relative to what's normal for you in your area.”

Pregnancy outcomes are just one risk to human health that researchers are linking to climate change caused by burning fossil fuels. Heat-related illnesses are on the rise. Some two billion people lack safe drinking water, according to the World Health Organization, which cautions that it can be challenging to accurately estimate the scale of climate-related health risks. Six hundred million suffer from food borne illnesses. Seven hundred and seventy million faced hunger in 2020.

“Climate change presents a fundamental threat to human health,” the says. 

As for a next step? Look at whether extreme heat is changing the frequency of preterm births. It could be a more difficult question because of the different ways countries record preterm births, Dahl said.

“Cutting fossil fuel emissions isn’t just good for the planet — it’s a crucial step toward protecting pregnant people and newborns around the world,” Bekkar said.