A major study has revealed that babies born from frozen embryo's through IVF may be more than twice likely to get childhood cancer.
Data taken from 1,085,172 children, from scientists at the Danish Cancer Society, showed that they were 154 per cent more likely to develop cancer than those who were born naturally from their mothers.
Among the children who were conceived through fertile mothers, 17.5 cases of cancer per 100,000 children were found. For frozen IVF babies, 44.4 per 100,000 cases of cancer were found.
All the children in the study were born between 1996 and 2012 and 3356 of them had been conceived through frozen-embryo transfer.
The link between childhood cancer, such as leukaemia, was only linked to frozen embryos and not IVF itself.
Experts in the study revealed in their findings that the risk is small and the study should not worry parents who have added freezing methods to conceiving children.
2217 of the more than one million children were diagnosed with cancer before the age of 20, 14 of them being frozen-embryo babies.
"No statistically significant association was found between use of any fertility treatment, any fertility drugs, IVF, or intracytoplasmic sperm injection and cancer in children," Dr Marie Hargreave, wrote in the Journal for the American Medical Association.
"However, children born after the use of frozen-embryo transfer were at an increased risk, mainly driven by an increased risk of leukaemia and sympathetic nervous system tumours."
Leukaemia was the most common single disease, with 648 cases, followed by lymphoma, central nervous system tumours, other nervous system tumours and other cancers.
The researchers also revealed that oestrogen, the female sex hormone, is an established risk factor for cancer.
Frozen-embryo transfer, the Danish researches said, was one of the only fertility treatments which included oestrogen in its treatment.
Denmark has one of the world's highest rates of IVF, where eight million children between 1978 and 2018 were conceived through assisted reproduction.