Hot weather increases pregnant women’s risk of giving birth early, and more babies could be born early as a result of global warming, researchers report today (December 2) in Nature Climate Change. The average reduction in gestational length is six days, they find.

“Increased exposure to hot weather with climate change is likely to harm infant health,” write coauthors Alan Barreca, an economist at the University of California, Los Angeles, and Jessamyn Schaller, an economist at Claremont McKenna College, in the study. That’s because early birth is tied to poorer physical and mental health later in life.

Reviewing county birth rates around the time of extreme heat waves in the United States from 1969 to 1988, the researchers estimated that roughly 25,000 infants per year were born earlier than their due dates as a result of heat exposure, and that the heat led to...

“More study needs to be done,” Mitchell Kramer, the chair of obstetrics and gynecology at Northwell Health’s Huntington Hospital in New York who was not involved in the study, tells HealthDay, “but certainly we must help protect pregnant women from extremes of heat as well as work on the causes of climate change.”

What causes pregnant mothers to have their babies earlier in hotter weather isn’t clear, but scientists have suggested heat leads to cardiovascular stress, which can induce pregnancy, or heat increases the levels of oxytocin, a hormone plays a role in labor, Time reports.

“There may even be a third cause,” Barreca tells Time, “which is loss of sleep. Minimum temperature on a hot day occurs at night, but it can still be hot enough to disrupt sleep, and that might be an important avenue to early birth.”

See “Sleeping for Two

Income and exposure to heat make a difference, the team notes. For instance, access to air conditioning, typically associated with higher income, cut early birth risk. And, expecting mothers in regions of the US where temperatures are typically high didn’t have as many early births as women who live where temperatures are cooler.

“Electrification and access to air conditioning should be a part of any effort to protect pregnant women and infants in developing countries,” Barreca tells The Guardian. “But developed countries, like the US or England, should be paying developing countries to electrify with renewable sources, like wind or solar, so we avoid producing more greenhouse gas emissions.” 

Ashley Yeager is an associate editor at The Scientist. Email her at ayeager@the-scientist.com.

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