- The research found that there were more than 25,000 heat-related early births in the US between 1969 and 1988.
- Early and premature births are linked with negative health outcomes for children, including respiratory problems.
- As the climate continues to warm, the number of days with extreme temperatures is expected to increase so the number of early births is projected to rise, too.
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On extremely hot days, women in the US give birth earlier than they naturally would otherwise.
That's the finding of a study published Monday in the journal Nature Climate Change. According to the research, the rate of childbirth in the US spiked by 5% on days when temperatures reached 90 degrees Fahrenheit (32 degrees Celsius).
So as the planet warms and days with extreme heat become more frequent, we're likely to see more early births, according to Alan Barreca, the lead author of the study and an environmental researcher at the University of California, Los Angeles.
Between 1969 and 1988, an average of 25,000 children were born early every year because of heat some as much as two weeks early. Barreca told Business Insider that "with climate change, we expect 42,000 additional births to deliver early per year in the United States by 2100."
That would bring the total number of early births to 67,000 by the end of the century a number Barreca found shockingly high.
"The most surprising thing to me is the number of births that are affected per year," he said.
Premature and early births are linked with negative health outcomes for infants and children, including a higher risk of respiratory problems.
A 'spike-then-fall pattern' in early births
For the study, Barreca and his co-author looked at 56 million births in the US between 1969 and 1988 and compared the dates and locations of those births with nearby weather events. The results showed that on days with temperatures above 90 degrees Fahrenheit, birth rates spiked by 5%. Then an immediate decline was observable in the following days and weeks.
"The spike-then-fall pattern over two weeks indicates that some deliveries are accelerated by two weeks," Barreca said.
On average, the human gestational period is 40 weeks. But according to the new research, 25,000 women in the US during the years studied gave birth an average of six days before that 40-week mark. All told, that adds up to 150,000 lost gestational days that US babies should have spent in the womb.
If carbon emissions continue unabated, most of the US could see 20 to 30 more days every year with temperatures above 90 degrees Fahrenheit within a few decades, according to the Union of Concerned Scientists . Another study predicted that the annual number of days with temperatures above 100 degrees Fahrenheit (38 degrees Celsius) will double by 2065, compared to the end of the 20th century.
That means far more babies are likely to come early.
"We predict more than 1 in 100 births will occur earlier than expected in the US by the end of the century," Barreca said.
The study authors offer two possible reasons why extreme heat causes an increase in early births: Either pregnant mothers get dehydrated, which causes increase in their levels of oxytocin (a hormone that regulates the onset of delivery), or the heat puts the mother under cardiovascular stress.
A 2016 study from the National Institutes of Health also found that women exposed to extreme heat for the majority of their pregnancies were more likely to deliver early.
Earlier births are linked with negative health outcomes
Premature babies, sometimes called "preemies," face a higher risk of health and development problems later in life. Babies born before the 37-week mark are three times more likely to develop attention deficit hyperactivity disorder (ADHD) than full-term, healthy-sized infants, according to a 2017 study .
Losing two gestational weeks doesn't shift babies into the "preemie" category, but it could still lead to negative health impacts, Barreca said.
A 2017 study revealed that the last two weeks of gestation are critical for the development of babies' lungs. Babies born at 37 or 38 weeks are more likely to have respiratory problems during childhood than full-term babies. According to the American College of Obstetricians and Gynecologists , babies born in week 37 or 38 have poorer overall health than their full-term counterparts.
Research has also found that babies born one week early are more likely to have lower test scores in childhood and lower incomes in adulthood.
Air conditioners could help
When greenhouse gases from the burning of fossil fuels enter the atmosphere, they trap more heat on the planet, causing Earth's surface temperatures to rise. If you envision the planet's temperature range as a bell curve, climate change shifts that entire curve towards the higher end of the temperature spectrum.
That puts a bigger chunk of days into the extreme-heat range.
"A warming of 1 degree Celsius, which is what we've seen thus far, can lead to a 10-fold increase in the frequency of 100-degree days in New York City, for example," climate scientist Michael Mann told the New York Times over the summer.
July 2019 was the hottest month ever recorded in history.
According to Barreca, one simple measure could lower pregnant mothers' risk of heat-related early labor: air conditioning. His study suggests that access to AC can reduce the impact of extreme heat on the rate of early births by 75%.
"Air conditioning is an already proven and highly effective solution," he said, adding, "every pregnant woman should have an air conditioner and enough money to pay for the associated electricity."
But there are a couple of obvious problems with that option. First, not all women worldwide have access to air conditioning.
Second, "running your air conditioner is expensive," Barreca noted. "The financial cost might be so high that people stop using air conditioners and have to suffer through extremely hot weather, which would be bad for prenatal health."
And there's a third issue: Generating the electricity needed to run air conditioning systems emits greenhouse gases. That contributes to the planet's rising temperatures, which further raises a pregnant mother's risk of early labor.
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