An extensive study by researchers has found that children born from frozen embryos appear to have a higher risk of cancer after birth.

A new study of more than 8 million children in Nordic countries suggests the possibility that children born after use of a fertility procedure known as ‘frozen-thawed embryo transfer’ may have a higher risk of cancer than children born through other means.

Assisted reproductive technology (ART) allows an embryo to be created from a human egg and sperm in a laboratory. A doctor may immediately transfer the embryo to the uterus, or, in a practice that is increasing worldwide, the embryo might be frozen and later thawed before implantation.

Prior research suggests that children born after frozen-thawed transfer may have higher short-term risk of certain medical issues than children born after fresh embryo transfer. However, potential long-term medical risks have been less clear.

To boost understanding, the University of Gothenburg, Sweden’s Nona Sargisian and colleagues analysed medical data from almost 8 million children in Denmark, Finland, Norway, and Sweden; 171,744 were born after the use of ART, and 7,772,474 were conceived spontaneously without the use of assisted reproductive technology.

Among those born after the use of ART, 22,630 were born after frozen-thawed transfer.

Statistical analysis of the data from national health registries showed that children born after frozen-thawed embryo transfer were at higher risk of cancer than children born after fresh embryo transfer and those without ART. When analysed as a single group (i.e. those born after frozen-thawed transfer and fresh embryo transfer), however, the use of any type of ART did not have an increased risk of cancer. The most common types of cancer seen in this study were leukaemia and tumours of the central nervous system.

The researchers emphasise that their findings should be interpreted with caution, since although the study was large, the number of children born after frozen-thawed embryo transfer who later developed cancer was low (48 cases), which could limit the statistical strength of the analysis.

Nonetheless, the findings may raise concerns about frozen-thawed embryo transfer. Future research is now needed to confirm a possible link between the procedure and increased risk of cancer, as well as any biological mechanisms that may underlie such risk.

Co-author Ulla-Britt Wennerholm said: “A higher risk of cancer in children born after frozen-thawed embryo transfer in assisted reproduction, a large study from the Nordic countries found. The individual risk was low, while at a population level it may have an impact due to the huge increase in frozen cycles after assisted reproduction. No increase in cancer was found among children born after assisted reproduction techniques overall.”

The findings have been published in PLOS Medicine.