Babies born from embryos frozen by IVF are at increased risk of cancer: study

Children born using frozen embryos may be at higher risk of cancer than those born by other methods, a large Nordic study suggests.

Although the absolute number of children with cancer was low, the researchers say their results should prompt fertility clinics to move away from the “freeze everything” approach until more is known about how freezing and thawing embryos can affect fertility. health of future generations.

It is estimated that almost one in 12 children in Europe today is born after fertility treatment, including in vitro fertilization (IVF).

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This type of assisted reproductive technology (ART) makes it possible to create embryos from a human egg and sperm in a laboratory and, just three days later, transfer them to the patient’s uterus.

But increasingly, IVF embryos are frozen for a few months – or years – before being thawed and implanted for pregnancy.

For their study, published in the open-access journal PLOS Medicine, researchers at the University of Gothenburg, Sweden, analyzed medical data on nearly 8 million children from Denmark, Finland, Norway and Sweden.

Of these, more than 170,000 were born after the use of ART, including 22,630 born after frozen-thawed embryo transfer.

The research found that children born after frozen-thawed embryo transfer had a 1.6- to 1.7-fold higher risk of cancer than those born after fresh embryo transfer and those born unaided by any fertility treatment.

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In absolute numbers, they are still very few: only about 2 in 1,000 children born after frozen embryo transfer would have cancer, compared with less than 1.5 in 1,000 children in the other two categories.

“In fact, this increase is quite modest. And that is quite reassuring for parents: Most of the children are healthy,” Christina Bergh, professor of obstetrics and gynecology at the University of Gothenburg , told Euronews Next (LON: NXT ). and co-author of the study.

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Children born after frozen-thawed embryo transfer had a 1.6- to 1.7-fold increased risk of cancer Canva

Why is it important? Nevertheless, the team say the results are noteworthy, as the number of children born via frozen-thawed embryo transfer is increasing and, in many countries, already exceeds the number of children born after fresh embryo transfers.

Previous research suggests that babies born after frozen embryo transfers also have an increased risk of macrosomia – or birth weight greater than 4 kg – which in turn has been associated with an increased risk of childhood cancer.

However, studies of any direct relationship between the practice of freezing embryos and the risk of childhood cancer have produced conflicting results.

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The researchers say this could be due, in part, to the limited size of such studies – since few children actually develop cancer – as well as differences in cancer registry practices between countries.

They say the main strength of their new study is the large sample size, which looks at populations born over a period of up to three decades in four Nordic countries known for their high-quality health records.

However, the authors caution that their new study cannot definitively link frozen embryo transfers with an increased risk of childhood cancer. They note that the data is observational and that other factors such as genetics, parental health before conception, and lifestyle cannot be excluded.

Leukemia and brain tumorsThe team compared the risk of cancer between children born after ART and spontaneous conception, between children born after frozen embryo transfer and fresh embryo transfer, and between children born after frozen embryo transfer. and a spontaneous conception.

Possible variables were taken into account, such as the stage of the embryo, maternal age, birth order, gender, birth weight, and whether the babies were singletons or multiples.Each type of childhood cancer has its own profile of risk factors, but many childhood cancers are thought to result from embryonic accidents and originate in the womb.Study authors

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Their analysis showed that children born after frozen-thawed embryo transfer had a higher risk of cancer than those born after fresh embryo transfer and those born without the help of ART.

However, when analyzed as a single group (including those born after freeze-thawed transfer and fresh embryo transfer), the use of any type of ART did not show an increased risk of cancer.

The most common types of cancer found in this study were leukemia and tumors of the central nervous system.

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“The reason for a possible increased risk of cancer in children born after FET [frozen-thawed embryo transfer] is unknown,” the study says.

“Each type of childhood cancer has its own profile of risk factors, but many childhood cancers are thought to derive from embryonic accidents and originate in the womb.”

The researchers stress that their results should be interpreted with caution because, although the study was large, the number of children born after frozen-thawed embryo transfer who subsequently developed cancer was low (48 cases), which could limit the strength analysis statistics.

Still, the results may raise concerns about frozen embryo transfer, and the team says future research is needed to confirm the possible link between the procedure and increased cancer risk, as well as the biological mechanisms that may underlie this. said risk.

What can cause an increased risk of cancer in “cryobabies”? There are three hypotheses, Bergh told Euronews Next.

It could be that the freezing procedure itself has an impact on cancer risk.

It could be an endometrial factor; something related to the fact that the lining of the uterus of women who receive a frozen embryo is usually much less stimulated by fertility treatments than those who receive a fresh embryo transfer.

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Indeed, women who have an embryo transferred within days of egg retrieval and IVF typically receive a hormone regimen lasting several weeks to increase the number of eggs they can produce and maximize the chances that at least one is fertilized.

Or it could have to do with the fact that embryos that survive the freeze-thaw process are the fittest to survive and “have more growth factors,” which could also explain higher birth weight, Bergh said.

“There could be a connection with hormones and growth factors causing the overgrowth,” he said. “Because cancer is kind of an overgrowth of certain cells, so there are some similarities between cancer and overgrowth.”

She said the findings shouldn’t alarm parents, but they should give pause to fertility clinics that prefer the “convenience” of frozen embryo transfers, as they can be scheduled outside of weekends and holidays. .

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In the United States, the rate of frozen embryo transfers has doubled since 2015 and now accounts for more than three-quarters of all embryo transfers, the study found.

“Our message is that you have to freeze (embryos) for medical reasons, not for other reasons,” Bergh said. Cases where patients are hyperstimulated after an IVF procedure are a valid reason to delay a transfer, she said.

“Our study shows that there appears to be an increased risk of cancer in cryobabies, but not in babies from fresh embryo transfers. And you have to be a little more careful with freezing approaches.”

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