Stifling heat isn’t the only reason it sucks to be pregnant during the dog days of summer.

Dogs born during summer months run a higher risk of heart and artery problems, according to a new study by researchers at the University of Pennsylvania.

The researchers combed through cardiovascular data for 129,778 dogs from more than 250 breeds for the study. They found that dogs born between June and August are at a higher risk of heart disease than those born during the rest of year.

The figures spiked to a 74% higher risk for dogs born in July than for dogs born in January.

Dogs born in April and May had the lowest risk of heart problems, 20% and 27% lower, respectively.

Interestingly, the difference in seasonal birth-month risk among breeds that are genetically predisposed to heart disease—such as boxers, dachshunds, and golden retrievers—was marginal.

And ironically, breeds not known to be genetically predisposed to cardiovascular disease—such as the American Staffordshire terrier, the English toy spaniel, and the Havanese—were found to be at increased risk of heart disease when born in the summer months.

Researchers say this suggests that the risk of heart disease may be dependent on birth season among all dog breeds.

Overall, dogs have a 0.3% to 2% risk of developing heart disease depending on breed.

What does this mean for veterinarians faced with a pregnant dog and a long, hot summer?

At the moment, not much—corresponding author Mary Boland, PhD, told NEWStat that it’s too early for pet care professionals to start instituting specific protocols based on a summer due date: “At this point, we are not recommending any change in treatment for pregnant dogs that are due to give birth during a high-risk month.”

Although it’s probably okay to err on the side of caution and close the windows during an ozone alert. Boland, an assistant professor of Informatics in Biostatistics, Epidemiology, and Informatics at the University of Pennsylvania’s Perelman School of Medicine, says summertime air quality could be the culprit: “A related study in humans appears to point to an air pollution mechanism.”

In that study, Boland’s team examined health data for 10.5 million people in three countries and found that people exposed in the womb to summer air pollution during the first trimester of pregnancy had a 9% higher chance of experiencing heart rhythm problems as adults.

The researchers note that because dogs have a two-month gestation period compared to nine months for humans, the window of in utero exposure to summer air pollution is roughly the same.

“The canine heart is a remarkably similar model to the human cardiovascular system,” said Boland. “Also, humans and dogs share their lives together and are exposed to similar environmental effects, so seeing this birth season–cardiovascular disease relationship in both species illuminates [the mechanisms behind it].”

Taken together, both studies support the idea that exposure to outside air pollution during pregnancy and at the time of birth may play a role in later development of heart disease in both dogs and humans.

But Boland also cautioned that, “future studies need to tease out the causal mechanisms underlying this association.”

Still, you might want to go ahead and close that window.

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