What are 'three-parent babies'? Inside the groundbreaking IVF trial changing lives

In a breakthrough hailed as a major leap forward in reproductive medicine, scientists in the UK have successfully used a new form of in-vitro fertilisation (IVF) to help mothers avoid passing on devastating mitochondrial diseases to their children.

The result? Eight healthy babies born with DNA from three individuals — their mother, father, and a donor — in what is being called the world’s first confirmed trial of its kind.

Published this week in the New England Journal of Medicine, the trial offers new hope to families affected by mitochondrial disorders, which are rare but incurable genetic conditions.

Mitochondrial diseases affect approximately one in every 5,000 births and can cause symptoms ranging from muscle wasting and seizures to organ failure and early death. In cases where a mother carries faulty mitochondrial DNA, traditional IVF screening may not guarantee a healthy embryo. The newly trialled technique, however, reduces the amount of mutated mitochondrial DNA by over 95% in most cases, offering a promising pathway for safe pregnancies. Still, the method — which involves modifying embryos at a genetic level — is not without controversy and is currently approved in only a handful of countries.

The method is called mitochondrial donation, and it involves replacing defective mitochondria in a mother’s egg with healthy mitochondria from a donor egg. Most of a person’s DNA — 99.9% — comes from the nucleus of the mother’s and father’s cells. But the mitochondria, the tiny powerhouses of the cell, also contain a small but crucial amount of DNA. If a mother’s mitochondria carry mutations, her child could inherit life-limiting diseases. In this technique, the nuclear DNA from the mother’s egg is transferred into a donor egg that has had its own nucleus removed but retains its healthy mitochondria. The embryo is then fertilised with the father’s sperm.

As a result, the baby ends up with DNA from three individuals — mother, father, and a female donor — though only about 0.1% of the baby’s DNA comes from the donor, and it doesn’t influence traits like appearance or personality. Scientists stress that the donor’s DNA plays no role in defining who the child is, similar to how a bone marrow transplant introduces donor DNA without altering a person’s identity.

This technique could provide a lifesaving option for families like those supported by Liz Curtis, whose daughter Lily died at eight months old from a mitochondrial condition. For parents who face a high risk of passing on such diseases, this IVF method could be the only path to having a healthy biological child. Out of 22 women in the trial, eight babies were born — all currently healthy. One child had a minor heart rhythm issue that was successfully treated, and a few showed early signs of a phenomenon called “reversal,” where defective mitochondria reappear, though not at disease-causing levels.

Experts like Dr Zev Williams from Columbia University call the trial a significant milestone in reproductive medicine, particularly because it expands reproductive choices for families previously told they had none. However, others note the modest success rate — just eight births so far — and caution that long-term effects are still unknown.

The use of mitochondrial donation is highly regulated and, in many countries including the United States and France, not allowed. Ethical objections include the modification of embryos at the genetic level, a concern that echoes longstanding fears about creating so-called “designer babies.” Religious groups also object to the destruction of embryos involved in the process. In the US, current legislation bans the FDA from even considering applications for such genetic techniques if they involve heritable modifications.

Proponents argue that the benefits far outweigh the risks, especially when the alternative is a predictable and preventable death. Julie Stefann, a French mitochondrial disease expert, says it comes down to a risk-benefit equation: “For a mitochondrial disease, the benefit is obvious. In the context of infertility, it has not been proven.”

All eight babies are currently healthy, but long-term follow-up is ongoing. Some babies showed signs of “reversal,” where faulty mitochondria re-emerge. It’s not fully understood yet.

Yes — in the UK, it’s limited to families at very high risk of passing on mitochondrial disease, with strict regulatory approval.

The UK remains one of the only countries in the world where mitochondrial donation is legally permitted under strict guidelines. So far, 35 patients have been approved for treatment. Researchers will continue to monitor the children born through this method to ensure their health remains stable and to better understand the rare phenomenon of reversal. Meanwhile, campaigners like Curtis see the development as a powerful beacon of hope for families who, until now, had few options.

The debate around altering human embryos is far from over, but for now, these eight children represent more than a scientific triumph — they represent the possibility of a future free from inherited suffering.

- with inputs from AP and AFP