The study, published in the medical journal JAMA on Tuesday, looks at babies born in Denmark, a country with one of the highest rates of assisted reproductive technology. In 2018, nearly 10% of all children were born through some form of fertility treatment.

Scientists already knew that children born with the help of fertility treatments faced increased health risks. The children are more likely to have a low birth weight, to be born prematurely and to have some birth defects, earlier studies have shown. Less is known about the long-term health consequences of such procedures, the authors said.

This study looked at data from the Danish Medical Birth Register, the Danish Cancer Registry, and the Danish Infertility Cohort that included 1,085,172 children born in Denmark between January 1996 and December 31, 2012. Of those children, 2,217 were diagnosed with cancer.

When scientists compared the number of children born to fertile parents with those born using some form of assistance, they noticed an elevated risk of childhood cancers in the cases where frozen embryos were used.

Specifically, the rate of childhood cancer was 17.5 per 100,000 for children born to fertile women and 44.4 per 100,000 for children born using frozen embryo transfer.

There were no statistically significant cancer associations with the other types of fertility treatments the scientists looked at in this study. The cancer risk wasn't any higher for children born to parents who used fertility drugs, IVF or intracytoplasmic sperm injection, among other methods.

"I think this is interesting and of potential concern, but you have to look at this study in perspective," said Dr. Jeffrey Goldberg, an OB-GYN and reproductive endocrinologist at Cleveland Clinic who was not involved with the study. "Fortunately, child cancers are pretty rare.

"It is something that warrants further evaluation, but there are a couple explanations."

Goldberg said it's important to keep in mind that the study started in 1996 and there have been a lot of changes in technology and in protocol in the lab and in the stimulation protocol since then, which could have an impact if the study were run again on more recent data.

Women who undergo this procedure using frozen embryos may also be at higher risk if they're older with older partners, and that can increase cancer risks. If the mother is obese, weight could also pose an increased risk that a child could develop cancer, Goldberg said.

"It kind of raises more questions than it answers," Goldberg said. "I think it was reasonable to ask the question in the first place in this study."

Most earlier studies did not find an association between cancer and frozen embryos. Although one prior study did see a link, all of the studies were based on much smaller patient populations.

This study had a large number of patients, but it is unclear if the results would be the same if scientists looked at patients from other countries with different racial and ethnic characteristics.

The study is also centered on retrospective data, so researchers weren't able to ask why there might be an elevated cancer risk with this procedure.

Goldberg said parents shouldn't worry.

"A 2.4 fold increase is clinically significant, but these are very small numbers, and this is based on older data going back to '96 and some of the things here may not be applicable now," Goldberg said. "I wouldn't want this to set off alarm bells."