Of all the impacts of climate change, extreme heat is one of the least visible – until, in retrospect, links can be made between sweltering temperatures and health consequences.

A new study of 53 million births over 25 years across the US has now found that early births become slightly more frequent during hotter, longer heatwaves.

Similar to the elderly, pregnant people, newborns, and infants are particularly sensitive to the effects of extreme heat, unable to cool themselves down as easily as the rest of us.

University of Nevada epidemiologist Lyndsey Darrow and colleagues analyzed national birth records between 1993 and 2017 across the 50 most populous metropolitan areas of the United States, a country where heatwaves have become 24 percent more intense and are occurring twice as often as in the 1960s.

Excluding rural areas not captured in the data, the researchers found that daily rates of preterm birth (between 28 and less than 37 weeks' gestation) and early-term birth (37 to less than 39 weeks) increased a small amount as local temperatures rose, particularly among lower socioeconomic groups.

Because an observational study like this can't elucidate direct causes, those are only associations, but they became stronger as temperatures increased and heatwaves stretched on, between four and seven days.

Regardless of whether heatwaves were defined using average temperatures, overnight minimums or daily maximums, the same effect was found.

Previous research on extreme heat has linked prolonged heatwaves with rises in hospitalizations, suicides, and deaths. Almost 62,000 people are estimated to have died from heat-related causes during the European summer of 2022, the hottest season for the region on record. By 2080, some estimates predict that major cities in the US and Australia could record at least four times the number of deaths from increasing temperatures alone.

Less often studied are the effects of extreme heat on pregnant people or those trying to conceive; we know far less about how their health is affected.

Darrow and colleagues' analysis is the largest study to date on extreme heat and early births, but it only encompasses the US.

Access to housing with reliable air-conditioning, being able to avoid physically demanding work in hot conditions, and pre-existing health conditions might have also modified pregnant people's individual risk of heat, the researchers noted.

It's not only brief heatwaves that impact pregnancies, either. Long stretches of above-average temperatures can also take a toll, research shows.

A 2020 meta-analysis of 70 studies from around the world found that while the evidence linking extreme heat and early births was "strongest and most consistent" for official heatwaves, cumulative heat over a person's entire pregnancy also increased their risk of having an early birth. In other words, hot days add up.

How hot temperatures feel also depends on the typical conditions of a local area, and how well the body can cope with excessive heat.

Although Darrow and colleagues' analysis didn't include humidity, a critical factor in how intolerable heat may be, they did look at hot days in the top 2.5 percent of average temperatures for local areas, not just official heatwaves lasting four to seven days. They found pregnancies in cool, dry areas will be affected as much as those in hot, humid parts of the US.

Researchers not involved in the latest study also point out that the lingering health effects for babies born early due to extreme heat are rarely studied.

"Although the association of exposure to extreme heat with adverse pregnancy and fetal outcomes is increasingly clear," writes Caleb Dresser, an emergency medicine physician at the Harvard T. H. Chan School of Public Health in Boston, and colleagues in a commentary, "the immediate and lifelong effects of prematurity are typically excluded from assessments of the effects of heat from both a health and economic perspective."

"By failing to include these implications of preterm and early-term births in public health assessments, we vastly underestimate the effects of heat on population health."

But with this new evidence, and a growing understanding of who is most vulnerable to extreme heat and how, Dresser and colleagues argue that health authorities, policymakers, and doctors are all better equipped to respond to heatwaves.

"The increasingly clear connection between heatwaves and harm to pregnant individuals and infants provides an important impetus to address root causes of our escalating exposure to heatwaves and invest in adaptive strategies to reduce their effects at the scale of cities, neighborhoods, and individual homes," they conclude.

The study has been published in JAMA Network Open.