Risk of Heart Defects Higher in Babies Conceived With I.V.F.

The birth defects were more likely, but still very uncommon, in infants conceived through certain fertility treatments, a large study found.

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.

It also did not make a difference whether the implanted embryos had been frozen for later use or the doctors implanted the embryos shortly after the eggs were fertilized in the lab.

The link between I.V.F. and birth defects of all kinds — including those that affect the muscles, genitals and gastrointestinal tract — has been well established in the scientific literature, said Dr. Jeffrey Kuller, a maternal-fetal medicine specialist at Duke Health.

The reason for that association isn’t clear, though.

One theory is that something about the I.V.F. process — which involves extracting a woman’s eggs, fertilizing them in a lab and then transferring the resulting embryo into the woman’s uterus — may be responsible.

Another theory, Dr. Kuller said, is that infertile parents have genetic differences that make their children more likely to have birth defects. For example, there is evidence that infertile men are more likely to have missing genetic material on the Y chromosome, which may be associated with some defects.

The high prevalence of twins and multiple births among women who undergo these fertility treatments may also contribute to the higher risk. Twins and multiples, however they were conceived, had the highest risk of heart defects, the study found.

Multiple births are more common with fertility treatments like I.V.F. because doctors sometimes transfer more than one embryo to increase the odds of a successful pregnancy. Even single-embryo transfers are more likely to divide into twins than in a naturally conceived pregnancy.

But as the dangers of twin pregnancies have become clearer and implantation has become more successful, the practice has become less common.

In more than 80 percent of procedures in 2020, just one embryo was transplanted, up from about 20 percent in 2011, according to the Centers for Disease Control and Prevention.

Dr. Wennerholm said she hoped that more recent data would show a drop in heart defects as a result of that shift. “I think that’s an important message to clinicians and to patients: You should go for a singular embryo transfer,” she said.

What to Know About I.V.F.

• In vitro fertilization can be daunting, but preparation and learning about the side effects can make it a lot easier. Our guide can help.
• There are still large gaps in our knowledge about how I.V.F. procedures affect women years later. Here’s why.
• Some couples, aware of the difficulties of conception and pregnancy, are choosing I.V.F. as a first option.
• Many insurance companies don’t cover I.V.F. treatments. But there are ways to ease the financial burden.
• For L.G.B.T.Q. couples, the path to parenthood can be long. One writer shared her absurd but ultimately successful experience using I.V.F. to become pregnant.
• Have you gone through an I.V.F. treatment? Tell us about your experience.