A significant Nordic study reveals that children born using frozen embryos may have a higher chance of developing cancer than children born using alternative procedures.
The researchers suggest their findings should lead reproductive clinics to deviate from a “freeze-all” approach until more is understood about how freezing and thawing embryos can harm the health of future generations, even though the number of children who actually had cancer was small. According to estimates, one in twelve children in Europe are currently born as a result of reproductive treatments, such as in vitro fertilization (IVF).
This kind of assisted reproductive technology (ART) enables the creation of embryos from human eggs and sperm in a lab and their transfer to the patient’s uterus as little as three days later.
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However, IVF embryos are now being kept for months or even years before being thawed and implanted for pregnancy on a regular basis.
Researchers from the University of Gothenburg in Sweden analyzed medical records from roughly 8 million kids in Denmark, Finland, Norway, and Sweden for their study, which was published in the open-access journal PLOS Medicine.
More than 170,000 of them were born following the use of ART, including 22,630 born following the transfer of a frozen-thawed embryo.
According to the study, babies born following a frozen-thawed embryo transfer had a roughly 1.6–1.7 times higher chance of developing cancer than babies born following a fresh embryo transfer and babies delivered naturally.
In terms of absolute numbers, it’s still very small: compared to less than 1.5 per 1,000 children in the two other categories, only about 2 out of every 1,000 infants born following a frozen embryo transfer will get cancer.
“This rise is actually fairly low. And it gives parents a lot of comfort: the majority of kids are healthy, “Co-author of the study and University of Gothenburg professor of obstetrics and gynecology Christina Bergh spoke to Euronews Next.
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Why is this important?
Though the number of children delivered utilizing frozen-thawed embryo transfers is rising and has surpassed that of children born using fresh embryo transfers in several countries, the research team thinks the findings are still significant.
According to earlier studies, infants born following frozen embryo transfers are also more likely to have macrosomia, or birth weight greater than 4 kg, which has been linked to a higher risk of childhood cancer.
Studies on any possible connection between the practice of embryo freezing and the incidence of childhood cancer, however, have produced mixed results.
As few children really develop cancer, the small sample size of such studies, as well as regional variations in the methods used to register cases of cancer, according to the researchers, may contribute to this.
They contend that the study’s enormous sample size, which examined populations born over a span of up to three decades in four Nordic nations renowned for their top-notch health registries, is its principal strength.
The authors do warn that there is currently insufficient evidence to conclusively link frozen embryo transfers to an elevated risk of pediatric cancer. They point out that as the data are observational, it is impossible to rule out other variables including genetics, parental preconception health, and way of life.
Brain tumors and leukemia
The team compared the risk of cancer in children born following ART with children born following a frozen embryo transfer with children born following a fresh embryo transfer, and with children born following a frozen embryo transfer with children born following spontaneous conception.
They considered potential factors like the stage of the embryo, the mother’s age, the kids’ birth orders, genders, birth weights, and whether they were singletons or multiples.
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According to their data, babies born following a frozen-thawed embryo transfer have a higher risk of developing cancer than babies born following a fresh embryo transfer and babies delivered naturally.
However, the use of any kind of ART did not demonstrate an elevated risk of cancer when examined as a single group, including those born following frozen-thawed transfer and fresh embryo transfer.
Leukemia and tumors of the central nervous system were the most prevalent kinds of cancer discovered in this investigation.
According to the study, “the cause for a probable increased risk of cancer in infants born following FET [frozen-thawed embryo transfer] is not known.”
Every type of juvenile cancer has its own risk factor profile, however many are believed to result from embryonic accidents and start in utero.
Although the study was big, the number of children born following a frozen-thawed embryo transfer who later developed cancer was low (48 cases), which could restrict the statistical strength of the analysis, the researchers stress that their findings should be regarded with caution.
The team warns that further study is required to confirm any potential associations between the process and an elevated risk of cancer and to identify any biological mechanisms that may explain such a risk. Nevertheless, the findings may raise concerns about frozen-thawed embryo transfer.
What could put “cryo-babies” at an increased risk for cancer?
According to Bergh, there are three theories.
It’s possible that the act of freezing itself affects the likelihood of developing cancer.
It might be an endometrial factor because women who get frozen embryos often experience significantly less stimulation from reproductive therapies than women who receive fresh embryo transfers.
In fact, those who have IVF and embryo transfer just a few days later have often only received a hormone regimen designed to increase the amount of eggs they can generate and increase the likelihood that at least one of those eggs will be fertilized.
The greater birth weights could also be explained by the fact that embryos that survive both the freezing and thawing processes are more likely to be suited for survival and “have more growth factors,” according to Bergh.
She speculated that hormones and growth factors might be related to overgrowth. “There are some similarities between cancer and overgrowth because cancer is a type of cell expansion.”
The research, she added, shouldn’t worry parents, but it should make fertility clinics reconsider their preference for frozen embryo transfers, which may be scheduled outside of weekends and holidays.
According to the study, the percentage of frozen embryo transfers in the United States has increased since 2015 and currently makes up more than three-quarters of all embryo transfers.
Our message is that you should only freeze (embryos) for medical needs, according to Bergh. Patients who have hyperstimulation following an IVF operation are a good candidate for a transfer delay, according to her.
“According to our research, babies born via cryotransfer appear to have a higher chance of developing cancer than babies born via fresh embryo transfer. And you need to be a little bit more cautious with freeze-all strategies “.
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