Male babies in the womb during the peak of the 1918-1919 flu pandemic were at increased risk of heart disease when they reached their 60s, 70s and 80s, researchers said.
In those men, the rate of heart disease was more than 23 percent higher than among those whose mothers were not pregnant during the pandemic's virulent phase in the fall of 1918, according to Dr. Caleb Finch, of the University of Southern California, and colleagues.
The main factor in the increase was a 32.7 percent rise in the rate of ischemic disease, Finch and colleagues reported online in the Journal of Developmental Origins of Health and Disease.
The 1918 pandemic was the most deadly on record, with mortality estimated as high as 100 million people worldwide.
But Finch said he and colleagues are concerned that even milder flu may have long-term consequences for the children of women pregnant during an outbreak.
"Our point is that during pregnancy, even mild sickness from flu could affect development with longer consequences," Finch said in a statement. "There is particular concern for the current swine flu, which seems to target pregnant women."
Other experts said the study appears to be solid, but cautioned against using it to draw conclusions about the current pandemic.
"I'd be extremely reluctant to infer anything about the 2009 swine flu outbreak from these findings," said Philip Alcabes of City University of New York.
Among other things, the study showed that heart disease was more common among all those who were in utero at the time, and not those whose mothers were actually infected with the flu, said Alcabes, who is the author of Dread: How Fear And Fantasy Have Fueled Epidemics From The Black Death To Avian Flu.
Since many women who were pregnant at that time either died or lost their babies, he said, "it could be that heart disease developed preferentially in offspring of women who were not infected, possibly because of some other exposure."
Dr. Howard Markel of the University of Michigan called the study "fascinating" but added that it only shows an association between later heart disease and in utero exposure.
"As compelling as the data is," he said, "there is no definitive proof that the latter caused the former."
But both Markel and Alcabes added that it's still important for pregnant women to avoid getting either seasonal or pandemic flu, if possible.
"This is why pregnant women are high on the priority list for vaccine once it is ready and they should get it," Markel said.
The study used data from the National Health Interview Surveys from 1982 through 1996, focusing on the 101,068 men and women who were ages 60 through 82 during those surveys.
Those people were born between 1915 and 1923, which allowed Finch and colleagues to compare the incidence of disease between the cohort in utero during the peak of pandemic and those whose mothers were pregnant with them before and after.
"They were able to compare babies born only months apart who differed only in whether they were in utero during the 1918 flu," said Jennifer Beam Dowd, also of City University of New York.
That makes the causal link relatively strong, she said.