Major heart defects are more common — but still rare — in babies conceived through certain fertility treatments, including in vitro fertilization, researchers reported on Thursday in the largest study of its kind.

The research, which included medical records of more than seven million Nordic children, also bolstered evidence that I.V.F. is associated with a small but significant uptick in birth abnormalities.

“It’s an increased risk, but the absolute risk is very small,” said Dr. Ulla-Britt Wennerholm, the senior author of the paper and a professor of obstetrics and gynecology at the University of Gothenburg in Sweden.

“I think that’s a reassuring finding, actually.”

The study focused on children born between 1984 and 2015 in Denmark, Sweden, Norway and Finland as a result of a class of fertility treatments called assisted reproductive technology, the most common of which is I.V.F.

The risk of a major heart defect was about 36 percent higher in this group than in children who were naturally conceived. But the defects were still uncommon: Less than 2 percent of infants conceived through A.R.T. were born with major heart defects.

The risk of heart defects didn’t change based on whether the parents underwent I.C.S.I., a procedure in which sperm is injected into an egg, or I.V.F., which allows the sperm to penetrate the egg naturally in a lab dish.