Can Extreme Heat Affect Your Pregnancy?

Key Points

• Expectant mothers are at risk of heat exhaustion.
• Extreme heat can lead to birth defects and death in pregnant women.
• Look for cooling stations, air-conditioned places, swimming pools or other bodies of water to help stay cool during a heat wave.

Americans may be aware of basic facts about pregnancy, such as avoiding raw fish, but who knew expectant mothers are extra vulnerable to heat stroke?

As the planet continues to warm and heat waves become increasingly common, access to air conditioning or cooling stations for pregnant women will be more important than ever to help prevent avoidable heat-related illnesses, birth defects and deaths.

What's the big deal about extreme heat?

Pregnant women aren't that delicate, but there's cause for concern for soon-to-be mothers during heat waves such as the one that dominated the summer of 2023. From the North Pole to southern Europe, July 2023 broke global weather records. NASA's Goddard Institute for Space Studies documented July 2023 as the hottest month on record.

The following heat-related facts should be sobering:

• Extreme heat kills more people in the United States than any other weather-related event, according to Scientific American.
• More than 600 people die in the U.S. each year due to extreme heat, according to the Centers for Disease Control and Prevention (CDC).

A deadly European heat wave killed more than 61,000 people in 2022, according to a study. The heat-related mortality rate associated with that heat wave was 63 percent higher among women than men.

Air conditioning, it seems, is no longer a luxury item but perhaps a necessity for survival at times.

How does heat exposure affect pregnant women and their babies?

Prolonged exposure to higher temperatures, especially when the heat index is over 90 degrees Fahrenheit, can cause pregnant women to deliver earlier. That increases the risk of congenital disabilities associated with premature deliveries.

Alan Barreca, a UCLA environmental economics professor, Jessamyn Schaller analyzed birth and climate data from 1969 to 1988 and found a disturbing heat-related trend. Their 2019 research suggested a strong correlation between high heat and pregnancies that ended in premature birth.

"We estimate that an average of 25,000 infants per year were born earlier as a result of heat exposure, with a total loss of more than 150,000 gestational days annually," the report stated. "Absent adaptation, climate projections suggest additional losses of 250,000 days of gestation per year by the end of the century."

Premature births can cause lower birth weight and lifelong health problems for the infant, including respiratory disorders such as asthma, Barreca said.

Sex in the summer? Yes, please:

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What are the health risks of heat waves while pregnant?

Pregnant women must remember they are more prone to heat-related illnesses, including heat exhaustion and deadly heat stroke, said Anna Cabeca, D.O., a Dallas-based OB-GYN with Carpathia Collective and the host of The Girlfriend Doctor podcast.

"When it's really hot in your environment, your body temperature spikes," she said. "And for pregnant women who are already more sensitive to heat, the overexposure to heat can increase the maternal body temperature and heart pulse, which in turn can lead to fetal heart tachycardia (or heart rate increase), and it can cause contractions to the uterus."

There's a reason why women aren't doing pregnancy yoga outdoors this summer. Time spent outside during a heat advisory is not worth the deadly risk to pregnant women or their babies.

The positive benefits of exercise during pregnancy are better attained in a cooler setting, preferably in an air-conditioned setting or swimming pool during the hottest months.

Expectant mothers' blood volume increases with pregnancy, Cabeca said. Pregnant women need to hydrate more than usual, even without the presence of a heat wave.

"And, as we know, excessive heat will use up the fluid in your body just to keep the body temperature cool, so if you are pregnant and have a heat wave, you have to drink water twice as much, add electrolytes and be very careful to not dehydrate," Cabeca said.

Are the body temperatures of pregnant women already hotter?

Pregnant women may feel extra hot, but their actual body temperature may not be higher. It's likely due to blood flow.

"The amount of blood increases when you're pregnant; that is because we store blood to feed the baby," Cabeca said. "Because of that, the blood vessels dilate more, bringing blood closer to the surface, which is why pregnant women feel warmer.

It may be 100 degrees or more outside, but that doesn't mean pregnant people have to shelter inside all summer.

"So it is not so much that their body temperature increases, but the way they can handle temperature changes because of all the physiological changes that are happening at that time."

A lack of temperature regulation puts pregnant women at high risk during heat waves.

Who is more at risk of poor maternal health outcomes due to extreme heat?

Several factors affect poor maternal outcomes caused by extreme heat. These include overall health, pre-pregnancy weight and age, Cabeca said.

Where you live and the kind of access you have to cooling areas can play a role, too.

"The environment and living conditions will also impact [you]," Cabeca said. "Do you have air-conditioning? Are you by a body of water where you can refresh a little?"

Why do lower-income people and communities of color experience more ill health effects during heat waves?

Low-income women, outdoor workers and Black or Hispanic expectant mothers are most likely to experience serious health effects associated with heat waves, according to a 2021 report by the National Partnership for Women and Families.

Black women are up to four times more likely than white women to experience heat-related maternal mortality linked to unequal exposure to heat and limited access to air-conditioned spaces, according to a 2021 study.

"People with low socio-economic status live in more hazardous places, fragile housing, no air conditioner, no clean air, no optimal hygienic conditions and limited access to healthcare," Cabeca said. "All that factors in your ability to resist and prepare for excess weather conditions, being heat waves or arctic cold fronts."

Do heat waves cause an increase in preterm births?

Pooled results from multiple studies found "preterm births increased by 16 percent during heat waves," according to the same National Partnership for Women and Families 2021 report.

"Excessive exposure to heat can be associated with early labor, stillbirth or babies with low birth weight," Cabeca said. "As explained previously, heat can put the body of a pregnant woman specifically under a lot of pressure, and with not enough air circulating, chances for dehydration and uterine contractions, it can be a recipe for a really bad experience and possible health scare."

Birth rates were 5 percent higher on high-heat days when the temperature exceeded 90 degrees Fahrenheit, Barreca's 2019 research suggested. The premature births occurred up to two weeks—the average was 6.1 days—before the mother's due date.

Does repeated exposure to high heat affect fertility?

High temperatures were associated with delayed conception, but birth rates typically rebounded in weeks and months following a heat wave.

"What I found was that extreme heat events led to fewer births nine and 10 months later … and that it's most likely explained by damage to sperm production," Barecca said.

That sperm damage is temporary, he added, but delayed conception carries with it additional heat-related concerns.

"If people tried to conceive in August and they end up being successful in December, when are they giving birth? They're giving birth the next fall," he said. "So women are pregnant when it's extremely hot out, and that's a health threat for them. They could end up delivering early if they get hit by a heat wave, sort of as they are in their later stages of the third trimester."

Why do my ankles swell more when it's hot outside?

It's common for expectant mothers to notice swollen ankles during hot weather, especially third-trimester swelling, Cabeca said.

"Pregnant women retain more water, and that water tends to work its way down the body throughout the day; hence, swollen ankles," she said. "In the heat, the body is in survival 'don't dehydrate' mode, so it tends to fight to hold on to the little water it can."

Lower extremities can swell during extreme heat, especially if people—pregnant or not—stand for an extended time.

Cabeca offers expectant mothers stretching tricks to beat the heat and swollen ankles.

"I always advise my pregnant ladies to put their feet up against the wall, or any kind of shape they are comfortable in, that puts their feet above their heart to stimulate circulation," she said.

How do you treat a heat rash while you're pregnant?

The last thing any pregnant woman wants during her third trimester, or postpartum, is a heat rash on her boobs. Breastfeeding is difficult enough without the presence of a heat rash.

Heat rashes can occur in humid conditions when sweat is trapped in the skin, according to Mayo Clinic. The rash can look like small blisters or larger bumps and may be itchy depending on the kind of heat rash.

Fear not, though, pregnant women. You have plenty of ways to stay healthy while pregnant during a heat wave, Cabeca said. Pregnant women can beat the heat and mitigate the risk of developing a heat-related illness, such as heat rash, by taking preventive measures before any sun exposure.

Pregnant women can try the following tips to avoid sunstroke or a heat rash, Cabeca said:

• Wear loose clothing.
• Apply cool compresses.
• Shower often (in cold or cool water).
• Turn on the fan.
• Turn down the air conditioning.
• Avoid sweat-inducing spicy food.

How can a pregnant woman beat the heat and stay safe during extreme heat?

Pregnant women are most vulnerable to heat stroke and heat-related birth defects, such as low birth weight, when they are dehydrated and exposed to higher temperatures, such as more than 90 degrees Fahrenheit, for a prolonged period of time.

Cabeca stresses the importance of hydration to her pregnant patients.

"Avoid being outside during the hottest hours of the day, stay in a cool air-conditioned environment, drink lots and lots of water, and eat healthy and balanced in order to support your hormones, which in turn will help with your body temperature," she said.

Access to a swimming pool is always a plus during a heat wave, pregnant or not.

"Be in the water as much as you can—pool, springs and beaches—and stay in the shade, wear a hat and loose clothing. Nothing that sticks to your body," she said.

What are the signs of heat exhaustion and heat stroke?

Heat exhaustion is often the warning sign of an impending heat stroke.

Symptoms of heat exhaustion, according to Mayo Clinic, may include the following:

• Heavy sweating
• Faintness
• Dizziness
• Fatigue
• Weak, rapid pulse
• Low blood pressure upon standing
• Muscle cramps
• Nausea
• Headache

Heat exhaustion may turn into a heat stroke, causing a life-threatening situation for the mother and baby. Seek medical treatment if you suffer from the symptoms of heat exhaustion.

When should a pregnant woman see a doctor?

Pregnant women showing signs of heat exhaustion should seek prompt medical attention.

Cabeca said she always encourages women to check with their physician if anything feels "out of sorts."

"Our bodies are ever-changing and go through massive fluctuations on a 'normal day,' let alone when pregnant, so it is always better to be safe than sorry," she said. "And sometimes that one doctor visit that you were not sure you should go to can make a massive difference in how you feel and how the baby feels."

The bottom line

"Yes, heat waves can be exhausting and very taxing on the body—and more so on the body of a pregnant woman for sure," Cabeca said. "But it is important to remember that many women around the world who live in excessively heated climates, such as the tropics or desert, get pregnant and have babies—and often without a major issue."

It may be 100 degrees or more outside, but that doesn't mean pregnant people have to shelter inside all summer. The trick to keep cool is to stay hydrated and limit the amount of time spent outdoors during the hottest part of the day.

"So it is also a matter of mindset, preparedness, common sense…and really trusting that your body is equipped with what it needs to adapt and adjust," Cabeca said.