As climate change increases average temperatures in many areas, the consequent increase in extremely hot days is likely to cause more infants to be born with congenital heart defects (CHD) in the U.S. in less than 20 years, a modeling study indicated.

Projections for 2025-2035 indicated that incidence of CHD could more than double in the Midwest, where the frequency of "extreme heat events" in summer was predicted to rise more than three-fold, according to Shao Lin, MD, PhD, of State University of New York in Rensselaer, and colleagues.

Substantial increases in both were also projected for the Arkansas-Texas region and the Northeast, the group wrote in the Journal of the American Heart Association.

Results also pointed to increases in certain CHD subtypes, including a 38.6% (95% CI 9.9%-75.1%) increase in atrial septal defects (ASDs) in the Northeast and a 34.0% (95% CI 4.9%–70.8%) increase in conotruncal CHD in the South during the spring, the investigators found.

Lin and colleagues were extrapolating from previous studies connecting exposure to high-heat days during early pregnancy to increased CHD risk as well as other adverse birth outcomes such as low birth weight and preterm birth.

"Maternal heat exposure during early pregnancy may directly cause fetal cell death or interfere with protein synthesis via heat-shock proteins and induce severe fetal malformations as observed in animal studies. As global temperatures continue to rise, more intense, frequent, and longer-lasting heat events are expected," the study authors noted.

"Our findings and geographic vulnerability maps provide useful information for policymakers in preparing and allocating resources for adapting to climate change," they concluded.

There are still considerable gaps in understanding the potential effect of climate change on heat-related CHD burden and maternal heat exposure. The current heat projection literature often focuses on common diseases like respiratory and cardiovascular disease, yet potential effects of temperature on pregnancy outcomes were not generally evaluated, the researchers highlighted.

Much of the existing heat-health projection literature either have relied on weather projections from low-resolution global climate models or have focused on conditions other than those relating to reproduction, the authors continued.

Clinicians, especially obstetricians, should advise pregnant women to be cautious about extreme heat exposure during the early stages of their pregnancy, noted Lin and colleagues.

"Pregnant women may be more susceptible to the adverse effects of early heatwave or extreme heat in spring. The increase in more frequent and longer duration of extreme heat events due to climate change would increase the demand for medical preparedness and early warning in spring," Lin told MedPage Today.

Lin's group gathered health data from the National Birth Defects Prevention Study, which assessed risk factors of major structural birth defects over an 11 year period in spring and summer. Patient information was collected from study centers in the Southwest (Utah), Midwest (Iowa), Southeast (North Carolina and Georgia), Northeast (New York), and the South (Texas and Arkansas).

The researchers relied on climate forecasts through 2035 from the Goddard Institute of Space Studies and NASA. They simulated variations in daily maximum temperatures based on geographic location, enhanced the temporal and spatial resolutions of the forecasts, and then determined the possible maternal heat exposure per region for summer and spring. They defined numbered heat exposure indicators.

The investigators combined the heat-CHD relationships determined during the baseline period of 1995 to 2005 with predictions of increased maternal heat exposure based on a projection period of 2025 to 2035 in order to come up with potential CHD burden estimates.

Their model indicated the prospect of more frequent exposure to springtime extreme heat events in the South would contribute to an 18.9% (95% CI 6.7%–32.6%) increase in ventricular septal defects, a 19.7% (95% CI 7.4%–33.5%) increase in conotruncal CHDs, a 12.3% (95% CI 5.9%–18.9%) increase in the number of total CHDs.

The researchers acknowledged significant limitations to their work, including their reliance on a simplified climate projection and on demographic projections that could change in the future.

"Another limitation is that it is difficult to project longer periods as many unknown factors such as population adaptability to heat and climate change severity, et cetera, may change over time," said Lin.