Heat waves can increase the risk of preterm births, research shows

The increase in frequency and intensity of heat waves across the U.S. can negatively impact our health before we’re even born, according to expanded research.

A study that was recently published in the medical journal JAMA Network Open looked at 53 million births from the 50 most populated metropolitan areas in the U.S. over a 25-year period and found a 2% increase in the rate of preterm births and a 1% increase in early-term births after multiday periods of extreme high temperatures.

There was already research that showed a correlation between high outdoor temperatures and preterm births, defined as delivery before 37 weeks of pregnancy. But this study specifically looked at the effects of heat waves on a much larger scale, comparing birth data with meteorological data and noting four consecutive days of high temperatures between the months of May and September for each particular metro area.

The research also found preterm — but not early-term — births were even higher among mothers who were 35 or older following a heat wave. An early-term birth takes place when a baby is born after 37 weeks, but before 39 weeks of pregnancy.

The negative effects of heat waves on perinatal health were not distributed evenly among the populations that were studied, however. The researchers noted that younger mothers from minority racial groups with a lower level of education had a higher rate of premature births following heat waves, at 4%.

Some of the contributing factors to that statistic could include expecting mothers having more or less access to air conditioning in their residences, and more or less financial resources to pay higher electricity bills brought on by air conditioning. The pregnant women in those vulnerable groups could also have jobs that require them to be outdoors and exposed to heat more frequently, the researchers said.

There are several biological reasons why heat can contribute to a mother going into labor early. Heat stress and dehydration reduce uterine and placental blood flow, which can affect contractions or hormone levels that control when labor is induced.

Heat stress can also cause oxidative stress, which releases heat shock proteins that may prompt a reaction that will trigger labor. Extreme heat may also trigger the premature rupture of membranes, leading to labor, the study said.

Preterm birth is a leading cause of infant death and long-term health conditions. And, since data shows heat waves are becoming more common and more severe across the globe, it puts a spotlight on the impact heat has on health — especially for vulnerable populations.

This research comes just as the U.S. Southwest is experiencing its first heat wave of the season, bringing dangerously hot, triple-digit temperatures much earlier in the year than usual.