How dangerous is extreme heat during pregnancy? What to know.

As the United States endures what has been a punishing summer for many, doctors and experts are increasingly worried about pregnant women, among the most vulnerable populations to extreme heat.

In the U.S. over 2 million preterm births (before 37 weeks) and nearly 6 million early-term births (between 37 to 39 weeks) between 1993 and 2017 were associated with heat waves, according to a recent study. The likelihood of delivering ahead of time rose after prolonged heat waves and higher average temperatures.

“Pregnant people are basically carrying around little space heaters in their body because the fetus that they’re carrying and the placenta generate a lot of heat internally,” said Juanita Constible, a senior heat solutions advocate at the Natural Resources Defense Council, an advocacy group.

Here’s what you need to know about risks for pregnant people experiencing extreme heat and tips on how to protect them.

What does extreme heat do to the pregnant body?

Pregnant women have higher demands placed on their circulatory system because their bodies produce more blood. Extreme heat can add to that strain and push a pregnant body beyond its capacity, which can lead to low birth weight and stillbirth in addition to preterm birth.

Dehydration: Sweating, the body’s mechanism to cool itself, can cause dehydration, which can lead to preterm or false labor contractions. Coupled with hotter temperatures and expansive heat waves, the “extreme heat can actually result in not just contractions, but preterm labor,” said Nathaniel DeNicola, an environmental health expert for the American College of Obstetricians and Gynecologists.

And once preterm labor starts, it can’t be stopped.

Preterm births are associated with severe risk for the baby’s health including low birth weights, difficulty feeding and breathing, heightened risk of infections, as well as risks for developmental delays later in life.

Decreased blood flow: Extreme heat also limits pregnant women’s ability to regulate their body in ways that can harm their growing babies. When the body starts to heat up, blood pulls away from the core and the uterus and rushes to the skin. The decrease in blood flow to the uterus and placenta – used to feed the baby – can result in low birth weight.

The risk for heat exhaustion, which can quickly escalate to heat stroke in pregnant women, is also heightened, DeNicola said.

“Any body is prone to heat exhaustion, and a pregnant woman’s is more so because there’s already so many demands on her system that are getting it closer to that threshold in the first place,” DeNicola said.

Harm from overnight heat: Pregnant women are also particularly vulnerable when overnight temperatures remain warm, providing little relief from daytime heat, said Ashely Ward, the director of the Heat Policy Innovation Hub at Duke University. According to a study that Ward co-authored, the risk of preterm birth increases as temperatures exceed 75 degrees Fahrenheit overnight.

The research showed that for every 2 degrees increase in overnight temperatures above 75 degrees, the preterm birth risk increased between 1% and 6%.

Who is at risk?

All pregnant women, especially those in their third trimester, must worry about the toll of extreme heat, DeNicola said. Black women and other women of color, who tend to have more dangerous pregnancies due to economic disadvantages and racial health disparities, are especially at risk, said Skye Wheeler, senior researcher in the women’s rights division at Human Rights Watch.

Black women bear an uneven burden in maternal mortality compared to other racial groups, dying at two to three times the rate of others. “This is a major reproductive justice problem and it’s a human rights problem in the U.S.,” Wheeler said. “Our concern is that the climate crisis is going to drive those unjust racial inequities deeper.”

One study found that college educated, Black mothers showed higher risk of preterm birth following a heat wave than other mothers, regardless of race. Another study that looked at the effects of heat found an association between increased preterm birth and neighborhoods where residents of color faced economic disadvantages.

Some Black women already face a range of disadvantages including poor housing, medical mistreatment from providers and a lack of green spaces. Historic racism and discrimination can also play a role in whether pregnant women of color have access to AC at home and at work, Wheeler said.

Extreme heat is a “new pressure on top of all the other pressures that are already on their bodies,” she added.

What can you do?

Hydrate. This is important for everyone during extreme heat but especially for pregnant women, who have more stresses on their bodies. “Pregnant women need to be drinking way ahead of thirst,” DeNicola said. “Once they find themselves feeling thirsty, they’re well behind.”

Stay cool in the evening. If you don’t have an air conditioner, Ward suggests taking a cool shower to lower your core body temperature. Soaking up to your ankles in cool water can also lower your overall body temperature, she said.

Prioritize air conditioning in your bedroom. Since overnight recovery from the heat is so important, if you only have one window unit, put it in your bedroom. “You spend a lot of time in there overnight, and those overnight hours are extremely important for the body to recover, especially for pregnant women,” Ward said.