A new study examining 53 million births over 25 years in the United States has found that early births become slightly more frequent during hotter, longer heatwaves. The study, conducted by University of Nevada epidemiologist Lyndsey Darrow and colleagues, analyzed national birth records between 1993 and 2017 across the 50 most populous metropolitan areas in the US.
The research reveals that pregnant individuals, along with newborns and infants, are particularly sensitive to extreme heat, as they cannot cool themselves down as effectively as others. This vulnerability is similar to that seen in the elderly. As heatwaves in the US have become 24 percent more intense and are occurring twice as often as in the 1960s, the researchers found a small increase in daily rates of preterm births (between 28 and less than 37 weeks’ gestation) and early-term births (37 to less than 39 weeks) as local temperatures rose. This effect was especially pronounced among lower socioeconomic groups.
Though this observational study does not establish direct causation, the associations between heat and early births grew stronger as temperatures increased and heatwaves lasted longer, from four to seven days. The effect was consistent regardless of whether heatwaves were defined by average temperatures, overnight minimums, or daily maximums.
Previous studies have linked extreme heat with increased hospitalizations, suicides, and deaths. During the European summer of 2022, an estimated 62,000 people died from heat-related causes. Projections suggest that by 2080, major cities in the US and Australia could experience at least four times the number of heat-related deaths.
The impact of extreme heat on pregnant individuals and those trying to conceive is less well-studied. Darrow and colleagues’ research is the largest to date on this topic, though it focuses solely on the US. Factors such as access to air conditioning, the ability to avoid physically demanding work in hot conditions, and pre-existing health conditions may also influence individual risk.
Long stretches of above-average temperatures, not just brief heatwaves, can affect pregnancies. A 2020 meta-analysis of 70 global studies found that cumulative heat exposure over the course of a pregnancy increases the risk of early births.
Darrow and colleagues did not include humidity in their analysis, which is a critical factor in how intolerable heat can be. However, they did consider hot days in the top 2.5 percent of average temperatures for local areas. Their findings suggest that pregnancies in cool, dry areas are as affected by heat as those in hot, humid parts of the US.
The lingering health effects for babies born early due to extreme heat are rarely studied. Caleb Dresser, an emergency medicine physician at the Harvard T. H. Chan School of Public Health, emphasizes the need to include the implications of preterm and early-term births in public health assessments. Excluding these effects leads to a significant underestimation of the impact of heat on population health.
With this new evidence and a growing understanding of vulnerability to extreme heat, health authorities, policymakers, and doctors can better respond to heatwaves. Dresser and colleagues argue that addressing the root causes of increasing exposure to heatwaves and investing in adaptive strategies at the scale of cities, neighborhoods, and individual homes is crucial. The study has been published in JAMA Network Open.
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