London - Babies born from frozen embryos may be at increased risk of cancer, research suggests.
A study of more than a million children found that those born via frozen IVF transfer were more than twice as likely to develop childhood cancer than those born via natural birth.
The babies were most at risk of developing leukaemia and tumours of the nervous system.
There was no increased cancer risk for IVF babies born using fresh embryo transfer, the study found.
Scientists stress that the absolute risk is still tiny, with only two cases of cancer for every 500 children who began life as frozen embryos. For every 500 born via natural birth or fresh embryos, there was one case.
British fertility experts believe families who have had babies by the frozen method have little to worry about.
But the scientists behind the research, from the Danish Cancer Society Research Centre in Copenhagen, insist the difference is large enough to warrant further study.
The study tracked 1.1 million babies born in Denmark between 1996 and 2012, following them for an average of 11 years.
The findings are "interesting and of potential concern, but you have to look at this study in perspective," Dr. Jeffrey Goldberg, a reproductive endocrinologist at Cleveland Clinic who was not involved with the study, told CNN.
"Fortunately, child cancers are pretty rare," and parents shouldn't worry about these findings, he said.
The scientists believe the extra risk is linked to the way freezing induces changes that may affect the way the child grows in the womb.