Warmer Weather Linked to Lower Birth Weight

Women in warm climates may want to consider ways to cool off this summer as new research draws a connection between higher temperature and adverse birth outcomes.

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Being pregnant during the height of summer can be uncomfortable, to say the least. But there may be more reason to seek relief from scorching temperatures than the discomfort of swollen ankles and a sweaty belly. New research as part of a collaboration between Ben-Gurion University of the Negev (BGU) and Harvard University is indicating that exposure to hot air temperatures during pregnancy can lead to lower birth weight and even preterm birth.

Scientists considered all live single births that took place in the state of Massachusetts from January 2000 through December 2008 (accounting for nearly half a million babies) and compared the information with stats from National Climatic Data Center monitoring stations, as well as traffic density input, while adjusting for other potential risk factors.

The risks of overheating

Published in the Environmental Health Perspectives journal, the findings note a correlation between ambient air temperature and birth outcomes—specifically, a .04 pound drop in birth weight in women who were exposed to air that was warmer by 47 degrees Fahrenheit.

While researchers looked especially closely at how strongly exposure during the third trimester was linked with birth outcomes, they did notice a negative association when exposure occurred throughout the pregnancies.

"During pregnancy, the body's core temperature increases during the first trimester and continues until delivery," says OB/GYN Sheryl Ross, M.D. at Providence Saint John's Health Center in Santa Monica, Calif. "In the third trimester, getting overheated and dehydrated can cause preterm labor and may increase the risk of lower birth weight babies. However, all trimesters of pregnancy have potential problems associated with them."

Although the newborn weight difference observed was less than one ounce—which doesn't sound like much in the grand scheme of things to some parents—the results are significant because low birth weight can have some serious health consequences, including perinatal morbidity and mortality. What's more, experts suspect that expectant women are more vulnerable to high temperatures due to a decreased ratio of surface area to body mass, increased fat deposits, and a diminished ability to cool off by sweating.

Great ways to cool off

Although Ross notes that the study's limitations might keep medical professionals from making explicit recommendations to their patients, she would not restrict expecting women from living or vacationing in warmer climates. Instead, she would stress the importance of drinking 8 to 10 glasses of water a day in order to avoid dehydration.

"In addition to staying hydrated, limiting exercising [in hot weather], staying in cool [or] air-conditioned environments, using fans, and dressing in loose and well-ventilated clothing are suggested," Ross says. "Luckily, our bodies have a self-regulating thermostat, also known as sweating, that prevents us from overheating ourselves and the baby."

Pregnant women should be especially careful when exercising in very hot or humid climates, according to Ross, which can lead to dehydration, fainting, dizziness, fatigue, and preterm labor. First, ask your health care provider if you're allowed to exercise during all stages of pregnancy. Then, opt for indoor activities like swimming and using an elliptical machine. Women might also consider taking a dip in a pool and treating themselves to facial mists to keep their internal body temperatures nice and low as the mercury rises in June, July, and August.

Interestingly, this intersection of pre- and postnatal health and the environment hasn't been widely considered by scientists until now. But global warming and climate change are making the potentially negative effects of rising air temperatures—especially during those important months of pregnancy—harder to ignore.