Nowadays, as people delay reproducing until their late 30s or well into their 40s, it is becoming increasingly common to require some form of assisted reproduction in order to conceive and carry a fetus to term. People who seek assisted reproduction generally assume it's been heavily studied in regards to efficacy and safety, but in actual fact this is not the case.
A study published in JAMA on December 10, 2019 evaluated the outcomes of 1,085,172 children born in Denmark after the use of some form of assisted reproduction. The methods used included frozen embryo transfer, in vitro fertilization, intracytoplasmic sperm injection, and hormonal treatment, and the primary outcome evaluated was developing cancer during childhood.
The study found that among these methods, frozen embryo transfer was associated with a significant increase in the number of childhood cancers that developed among the live-born children. The study had a mean follow-up of 11.3 years and found that 44.4 children per 100,000 frozen embryo transfer births developed childhood cancer versus only 17.5 cases of childhood cancer per 100,000 natural conceptions/births.
None of the other methods used to assist reproduction were found to be associated with any increase in the risk of childhood cancer.
Most of the cancers were various types of leukemia, which is the most common type of cancer that affects children. Currently, around 80% of children diagnosed with leukemia survive for at least five years and depending on the exact type of leukemia, many children can be cured. However, the treatments can cause lifelong health problems for survivors of childhood cancers, such as infertility and inducing the development of other cancers that arise during mid-life.
Most cases of childhood cancer can be considered to be birth defects caused by genetic changes that occur during fetal development. This study's results therefore suggest that growing an embryo in vitro and then stopping its development by freezing it and storing it, sometimes for a prolonged period of time, is caused genetic damage.
The results of this study should cause IVF clinics to consider re-evaluating their protocols. Some clinics routinely freeze all embryos for a period of time to allow the woman's body to recover from the hormone treatments used to collect the eggs. Some studies have been published indicating that using a recovery period before embryo transfer is less likely to result in a premature birth or an underweight baby, and slightly increases the live birth rate relative to the use of a fresh embryo.