Research recently published in the journal Nature Climate Change found that extreme heat causes about 25,000 American babies to be born earlier each year.

On days that reach at least 90 degrees, birth rates tick up, the scientists discovered. Then, they dip back down as the temperature cools.

That pattern is how the researchers, economists Alan Barreca of UCLA and Jessica Schaller of Claremont McKenna College, knew the births weren't added; they'd just been moved up.

How early is heat moving up delivery dates?

The study revealed that each affected birth came up about six days short of where it would have landed, adding up to more than 150,000 days of lost gestation across the country every year.

Those final weeks matter. Babies born early are more likely to need time in the NICU, because the lungs and brain are still developing right up to the due date.

Another study reinforced the link — it was published in JAMA Network Open in 2024. It analyzed more than 53 million U.S. births across the 50 biggest metro areas. After four straight days of extreme heat, preterm and early-term births both rose, and the effect grew the longer and hotter the heat wave ran.

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That study also found the burden isn't shared evenly. The increases were sharpest among younger mothers and minority women. These are groups that also tend to have less access to prenatal care, and the single best protection from the heat:

Crank up that AC

In hotter regions and better-cooled homes, the effect of a scorching day was noticeably weaker, which points to who is most exposed when the temperature climbs and the AC isn't efficient.

Scientists are still working out exactly why heat does this. The leading ideas involve dehydration reducing blood flow to the uterus, along with heat-related stress on the placenta.

However, the trajectory is clear. Without broader access to cooling and other adaptations, Barreca and Schaller project the number of heat-driven early births could climb toward 42,000 a year by the end of the century.

For now, the advice for pregnant people during a heat wave is the same advice that protects everyone else, just with higher stakes: stay cool, stay hydrated and don't tough it out.