Researchers analysed the rates of childhood cancers in a cohort of over one million children born in Denmark between 1996 and 2012. Of those, 3356 were conceived using frozen embryo transfer, where embryos are frozen after fertilisation and later thawed before being transferred to the woman's uterus. Across all births, they found that childhood cancer was very rare, with 2217 children diagnosed with cancer in total – a rate of around 1 in 500. This rate approximately doubled for the children born following frozen embryo transfer, of whom 14 developed cancer.
The authors of the paper published in the Journal of the American Medical Association emphasise that their findings are 'considered exploratory' and 'should be interpreted with caution'.
'It is important to stress the fact that the increased risk is very small for the individual as childhood cancer is very rare,' said first author, Dr Marie Hargreave from the Danish Cancer Society Research Centre in Copenhagen, Denmark.
The most frequently developed cancers were leukaemia and sympathetic nervous system tumours, which are among the most common types of childhood cancer. The authors did not find any association between IVF using fresh embryos and increased prevalence of childhood cancers. They propose 'the freezing and thawing of embryos, use of cryoprotectants [chemicals used to protect the embryos during freezing], and dissimilar protocols for the use of fertility drugs' as possible explanations for the difference.
'It should also be noted that [during] the time period that relates to this study [an] older method of freezing was used,' said Professor Abha Maheshwari, honorary professor and director of Aberdeen Fertility Centre, University of Aberdeen, who was not involved in the study. 'Freezing techniques have improved so much in recent years, so this may not apply with newer techniques.'
Professor Daniel Brison, scientific director of the department of reproductive medicine at the University of Manchester, who was not involved in the study also pointed to a recent UK study which showed that 'frozen embryo transfers are also associated with potential health advantages such as birth-weight and early child growth patterns which are closer to naturally conceived children.'