The children born of frozen embryos and transferred to the mother may have a somewhat higher risk of cancer than those born by spontaneous conception or by transfer of non-frozen embryos.

This is suggested by a large-scale study published in the specialized media PLOS Medicine, Although the authors caution that these results must be interpreted with caution.

Large cohort study

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The transfer of frozen embryos has spread in recent years in the field of assisted reproduction due to the advantage that it offers over other methods such as non-frozen embryo transfer (better embryo survival and greater number of live births).

However, some concerns had already been raised in this regard and, in fact, certain investigations had pointed to a higher incidence of child cancer in children born by this method.

In this case, the authors set out to investigate this question with the largest cohort study conducted to date. To do this, they took data from more than 170,000 children born through assisted reproduction, including more than 22,000 born from frozen embryos, and about 7.7 million children born through spontaneous conception in Denmark, Norway, Sweden and Finland.

A small but significant difference

After a median follow-up of 10 years, they found that the incidence rate of cancer diagnosed before the age of 18 was 16.7 per 100,000 people each year for those born spontaneously and 19.3 per 100,000 people per year for those born through assisted reproductive technology.

However, looking specifically at the incidence of those born after frozen embryo transfer, they found that there was a much larger difference: in this demographic, the rate was 30.1 per 100,000 people each year. Adjusting for factors such as macrosomia, birth weight, or major birth defects only marginally influenced the association.

In addition, looking at specific types of cancer, they highlighted that children born after frozen embryo transfer had more than twice the risk of developing leukemia compared with those born by non-frozen embryo transfer or those born by spontaneous conception.

Limitations

Still, the authors caution these results have certain limitations. On the one hand, the low number of registered cancers, in any case, can statistically distort the risks; on the other hand, there is no known reason why children born this way might have an increased risk of cancer.

For this reason, they believe that it is important to clarify whether these statistical differences are explained by the procedure itself, by other different factors, or by simple chance. This is not easy, since (fortunately) the incidence of childhood cancer is very low.

References

Nona Sargisian, Birgitta Lannering, Max Petzold, Signe Opdahl, Mika Gissler, Anja Pinborg, Anna-Karina Aaris Henningsen, Aila Tiitinen, Liv Bente Romundstad, Anne Lærke Spangmose, Christina Bergh, Ulla-Britt Wennerholm. Cancer in children born after frozen-thawed embryo transfer: A cohort study. PLOS Medicine (2022). DOI: https://doi.org/10.1371/journal.pmed.1004078.