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February 21, 2019

Hot, sticky weather is uncomfortable for anyone. But for pregnant women, the summer heat can be especially dangerous.

That’s because getting too hot or dehydrated can bring the risk of pregnancy complications, experts say.

Dehydration “can create a lot of potential problems” for pregnant women, says Dr. Saima Aftab, medical director of the Center for Fetal Care at Nicklaus Children’s Hospital in Miami. As a general rule, doctors caution pregnant women to avoid being in “any situation where they become too hot,” Aftab told Live Science.

High heat and early pregnancy
One concern is that hyperthermia, or an abnormally high body temperature, early in pregnancy may increase the risk of birth defects. In particular, there is evidence that women who experience hyperthermia in the first six to eight weeks of pregnancy are at higher risk of having babies with defects of the brain or spinal cord (called neural tube defects), such as spina bifida, Aftab says.

Women may develop hyperthermia from fever, exposure to excessive heat or even immersion in a hot tub. In fact, the Mayo Clinic recommends that women spend no more than 10 minutes in a hot tub because of the risk of hyperthermia.

However, Aftab notes that the risk of birth defects associated with hyperthermia is mostly limited to the first eight weeks of pregnancy. In other words, a temperature spike that occurs after eight weeks of pregnancy is unlikely to increase the risk of birth defects.

And even if women do experience hyperthermia early in pregnancy, the risk of neural tube defects remains low, Aftab said. (According to the Centers for Disease Control and Prevention, about 1,500 babies are born with spina bifida in the U.S. each year – about three cases per 10,000 births.)

Risk of dehydration
Women’s bodies go through many changes during pregnancy, including changes in the way their bodies handle fluids and control temperature. As a result, they may become a little more prone to dehydration or may be more likely to show symptoms of dehydration than they would if they weren’t pregnant, Aftab says.

Symptoms of dehydration can include dizziness or lightheadedness, which can be dangerous if those symptoms cause a pregnant woman to fall, Aftab says. (Falls during the late second and early third trimesters can be harmful to both mother and baby, leading to complications such as loss of amniotic fluid, according to the Mayo Clinic.)

In addition, dehydration causes the brain to produce a hormone called vasopressin (also called antidiuretic hormone), which triggers thirst. But this hormone is similar to oxytocin, the hormone involved in stimulating uterine contractions, Aftab says. Therefore, dehydration in the third trimester of pregnancy may trigger intermittent contractions because vasopressin acts similarly to oxytocin, Aftab says.

According to the University of Texas Southwestern Medical Center, these aren’t “true” labor contractions and can often be treated with hydration. But it’s possible that the contractions could trigger premature labor, Aftab said. Dehydration that continues into labor is also dangerous because it can lead to complications such as low blood pressure and hypovolemia, she says.

For all these reasons, it’s important for pregnant women to stay hydrated. Aftab said, “We know that not a lot of good things happen” if a mother is dehydrated during pregnancy.

Some studies have even linked warm weather in certain areas to an increased risk of preterm birth, stillbirth and low birth weight babies in the population. For example, in a study published last month in the American Journal of Epidemiology, researchers used a population of more than 2 million babies born in California between 1999 and 2013 to examine whether exposure to certain temperatures during pregnancy was linked to the risk of low birth weight (less than 2,500 grams or 5.5 pounds). They found that for every 10-degree increase in temperature above 60 degrees Fahrenheit (15.5 degrees Celsius), the risk of low birth weight increased by 16 percent.

The study only found an association, not a clear cause-and-effect relationship, nor did it consider whether there were ways for the mother to reduce her exposure to warmer temperatures, such as through air conditioning. Therefore, more research is needed to confirm this finding.

However, the study “adds to the growing body of literature suggesting that pregnant women and their fetuses are a vulnerable population following environmental heat exposure,” the researchers wrote.” Pregnant women should be categorized as a vulnerable subgroup for additional precautions during heat warnings,” the study concluded.

Aftab said pregnant women should try not to stay in the sun too long on hot days and should avoid direct exposure to the sun in the shade. To stay hydrated, the Cleveland Clinic recommends that pregnant women drink 10 to 12 glasses of fluids a day.