An estimated 25,000 babies per year were born prematurely in the United States over a 20-year span due to heat waves, according to a study last week in the journal Nature Climate Change.
The research “found that on days when temperatures soar past 90°F/32.2°C, birth rates increase 5%,” CNN reports . “Birth rates are also elevated on the following day, perhaps because labour and delivery continued into the next day, the study said. After temperatures drop, birth rates decrease.”
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The 5% shift added up to 25,000 premature births and 150,000 gestational days lost per year, with some of the babies born as much as two weeks early, the study found.
Co-authors Alan Barreca, an associate professor of environmental economics at UCLA’s Institute of Environmental Sustainability, and Claremont McKenna College economist Jessamyn Schaller told  Salon they chose to study the 56 million births between 1969 and 1988, a time before vital statistics systems became more cautious about the data they made public. “We realized that the way people had worked at studying the health impacts of extreme heat in third trimester needed a new look,” Barreca said. “So, we came up with an approach where we said, ‘OK, after an unseasonably hot day, how any more births do we see?’”
But then they took it a step farther. “The subtlety of our research is that we also looked after the heat wave, to see if there was a drop in birth, and that tells you how much earlier the births occurred,” he said.
Both news reports trace the various factors, from elevated oxytocin levels to higher cardiovascular stress to lack of sleep during extreme heat waves, that could trigger premature labour. “Our bodies work harder when it’s hot outside,” Barreca told Salon.
“Previous studies have found that mothers face an increased risk of preeclampsia, hypertension, and other health problems with higher temperatures,” CNN adds. “During a baby’s last weeks in the womb, there’s dramatic maturation in the brain and rapid physical growth. Studies have shown that babies born early have a higher risk of diseases such as asthma, higher risk of developmental delays, and greater risk the child will need to be hospitalized early on in life.”