Rising temperatures may lead to more premature births, putting greater numbers of children at risk of long-term health problems and disabilities, according to a study published Monday in the journal Nature Climate Change.
The study suggests that high temperatures — particularly, ones at or exceeding 90 degrees Fahrenheit — may trigger as many as 25,000 early births each year in the United States.
That number is likely to increase by an additional 42,000 premature births annually in the coming decades as global warming causes hot days to become hotter and more frequent, say the study’s authors.
“We predict more than 1 in 100 births will occur earlier than expected in the U.S. by the end of the century,” said Alan Barreca, one of the authors of the study and an economist at the Institute of the Environment and Sustainability, University of California-Los Angeles (UCLA), in an interview with the science and technology website Space Daily.
“That number may seem small, but that’s much higher than the risks of getting into a car accident,” he added.
A major health concern
Premature births — ones that occur at or before the 37th week of pregnancy — are considered a major public health issue. They are the leading cause of infant deaths in the United States. In addition, babies who survive a premature birth are at greater risk of developing a wide range of problems than are full-term babies (born at 39 to 40 weeks). Those problems include lung damage, heart problems, digestive problems, brain hemorrhages, infections, vision loss, hearing loss and cerebral palsy.
In 2018, the premature birth rate in the United States rose for the fourth year in a row. One in 10 American babies are now born too early, according to the Centers for Disease Control and Prevention (CDC).
Experts don’t understand all the reasons for why some babies are born prematurely, but several risk factors have been identified. Pregnant women who have an existing chronic medical condition, such as high blood pressure or diabetes, are more likely to go into labor early, for example, as are those who develop certain infections. Also at risk are women who are underweight or overweight or who smoke or use illicit drugs.
This new study suggests high ambient temperatures should be added to that list.
For the study, Barreca and his co-author, economist Jessamyn Schaller of Claremont McKenna College, analyzed data on 56 million U.S. births from 1969 to 1988. The researchers carefully matched each birth date to the daily temperatures in the counties where the babies were born. (They had to use an old dataset because after the 1980s, privacy concerns limited the details from birth data that could be released publically, including to researchers.)
The analysis revealed that on days when temperatures reached or exceeded 90 degrees, birth rates were 5 percent higher than on days when temps were between 60 and 70 degrees.
The birth rates fell, however, in the days and weeks following those 90-degrees-or-higher heat waves. That finding suggests, say the researchers, that many of the births on the hot days were premature.
The researchers calculated that births on days when temperatures soared into the 90s or beyond occurred an average of 6.1 days earlier than they would have otherwise, and in some cases occurred up to two weeks earlier.
“That’s enough to take somebody from what’s considered to be a pretty healthy pregnancy into a ‘we are somewhat worried’ pregnancy,” said Barreca, in a released statement.
Interestingly, the effect of high temperatures on premature births was less pronounced in areas of the U.S. where hot weather is more common — in, say, Mississippi rather than in Minnesota.
“People living in hotter climates could have acclimatized through natural physiological changes, adoption of cooling technologies or changes in building designs and urban planning, among other things,” the researchers speculate.
The effect was more pronounced, however, among black women and women living in poorer counties throughout the country — perhaps because those women are less likely to afford air conditioning, say the researchers.
Exact cause unknown
As Barreca and Schaller point out in their paper, there are several possible explanations for why extreme heat triggers more premature births. Such heat puts stress on the mother’s cardiovascular system, which may lead to an earlier delivery. Heat also increases levels of the hormone oxytocin, which plays a key role in regulating the onset of labor and delivery.
This study comes with caveats, of course. Most notably, the study is observational, which means it can show only an association between high temperatures and premature births, not a direct cause-and-effect relationship.
Still, the findings are worrisome, for they support other, smaller studies that have uncovered a link between high temperatures and an increase risk of premature birth. They’re also troubling because they underscore the myriad forms of harm that global warming is going to exact on human health.
Barreca and Schaller say that because air conditioning could help reduce the risk of premature births, communities need to implement policies that make air conditioning more affordable.
But those policies must also make air conditioning more energy efficient, they add. Most current systems of cooling air contribute to climate change by emitting greenhouse gases called hydrofluorocarbons — further raising pregnant women’s risk of early labor.
FMI: You can access a read-only version of the study through the Nature Climate Change website. To download the study, you’ll need a subscription.