(NewsNation) — A new study has found that heat waves increase the risk of early births and that extreme temperature events are harmful to fetal health and pregnancy in the U.S.

The research published in the Journal of the American Medical Association Friday studied 25 years of birth data and 53 million births and found that rates of premature and early-term births slightly increase after heat waves, particularly among socioeconomically disadvantaged subgroups.

Researchers found that after four straight days of extreme heat temperatures relative to the area, there was a 2% higher chance of babies being born before 37 weeks, which is considered premature, and 1% chance higher for early-term births, or at 37 or 38 weeks. 

The study also found that this risk was slightly higher among younger mothers, those with less education, lower socioeconomic levels and Black and Hispanic mothers. 

“Most pregnant patients are going to have access to air conditioning and ways to avoid those extreme temperatures, and the fact that we still see an uptick in the rates of these outcomes suggests that there are pockets of individuals who are having a much stronger response to these actual heat exposures because for whatever reason, they aren’t escaping those hot temperatures,” Lyndsey Darrow, an epidemiologist at the University of Nevada Reno who co-authored the study, told NewsNation. 

Darrow said that populations who either don’t have access to air conditioning or might not be using it because of the expense are experiencing these extreme temperatures and are driving the overall average response to the increase in the rate of early births. 

The study examined data from 1993 to 2017 in the largest U.S. metropolitan areas, including both urban and suburban areas. 

Preterm birth is the most common cause of death for infants, and long-term health implications include respiratory, cognitive and behavioral outcomes.

“There’s enough evidence at this point to suggest that these exposures can have effects in pregnancy, and that is a good cautionary approach for clinicians and their pregnant patients when we’re having these heat wave events,” Darrow said. 

One of the ways heat can bring about preterm labor is that “heat stress and dehydration reduce uterine and placental blood flow, possibly affecting uterine contractility and/or hormone levels governing the induction of labor,” according to the study. 

Extreme and prolonged heat waves are one of the effects of climate change, which are becoming increasingly frequent, according to the Environmental Protection Agency. 

The rising temperatures are causing a host of heat-related illnesses, emergency room visits and deaths, the agency noted.

Heat waves are occurring more often than they used to in major cities across the United States, according to the EPA. Their frequency has increased steadily, from an average of two heat waves per year during the 1960s to six per year during the 2010s and 2020s.