Healthy children born after mitochondrial donation in the UK

Eight children have been born in the UK following mitochondrial donation. All eight children have made normal developmental progress, and none of them show any sign of mitochondrial disease.

Nineteen women had IVF with pronuclear transfer (PNT), one of two mitochondrial donation techniques whose use in treatment is permitted by UK law. Clinical pregnancies were confirmed in eight of these women, of whom seven have had a live birth (one of the women had twins), while an eighth pregnancy is ongoing. The eight children were all born healthy and were all monitored closely during the first 18 months of life, according to two research papers from a team based in Newcastle-upon-Tyne. The two papers are published in the New England Journal of Medicine, accompanied by two further commentary articles.

The families who received the mitochondrial donation treatment remain anonymous. The mother of one of the eight babies said: 'As parents, all we ever wanted was to give our child a healthy start in life. Mitochondrial donation IVF made that possible.' Another of the mothers added: 'The emotional burden of mitochondrial disease has been lifted, and in its place is hope, joy, and deep gratitude.'

The team in Newcastle encompasses research scientists, clinicians and embryologists. This group pioneered PNT (see BioNews 444, 554, 641, 644 and 682), and theirs was the first clinic licensed by the UK fertility regulator – the Human Fertilisation and Embryology Authority (HFEA) – to offer such treatment.

PNT and PGT

One of the new research papers focuses on the embryology aspects of the treatment. Thirty-two women received HFEA approval for treatment with PNT, of whom 25 underwent egg retrieval. Of these 25 women, 22 had their eggs treated with ICSI. For three women, ICSI was unsuccessful, and the researchers noted that the fertilisation rate for eggs from women with a high proportion of unhealthy mitochondria (thereby making these women eligible for mitochondrial donation) was lower than the fertilisation rate for other women in the study.

PNT was used on the fertilised eggs of the 19 women for whom ICSI was successful. Of these 19 women, 18 had embryos transferred and/or frozen for later use. Seven of these women have given birth so far, and an eighth woman is pregnant.

The research paper also discusses a separate group of women at risk of transmitting mitochondrial disease, who were treated with preimplantation genetic testing (PGT) – a more established technique – rather than with mitochondrial donation. In these circumstances, PGT is only feasible for women who have a mixture of healthy and unhealthy mitochondria (such a mixture is known as 'heteroplasmy'), and it works by trying to select embryos for transfer that have a relatively low proportion of unhealthy mitochondria.

Thirty-nine women in this group had eggs that were treated with ICSI, of whom 31 women had embryos transferred and/or frozen for later use following selection via PGT. This PGT pathway has resulted in 18 live births to date, and none of the 18 children show any sign of mitochondrial disease.

The fertility pathway for families affected by mitochondrial disease

The other new research paper outlines the broader care pathway – how families affected by mitochondrial disease access fertility treatment, and what options are available – and discusses clinical outcomes, including the health of the eight children born following PNT.

All eight of these children were healthy at birth and are developing normally, meeting expected milestones. Three of them experienced health issues following birth, but these issues do not appear to be related to the mitochondrial donation, and in all three cases the issues were treated successfully. The most severe case was a child who had a cardiac arrhythmia and elevated blood fat levels, and in that instance the child's mother also had elevated blood fats during her pregnancy.

First author Professor Bobby McFarland, director of the NHS Highly Specialised Service for Rare Mitochondrial Disorders, said: 'We believe the follow-up process we have put in place is thorough, since it allows us to detect and review even minor health conditions in children born after pronuclear transfer such as a urinary tract infection.'

Carryover and reversion

One of the concerns that surrounds PNT is that some of the unhealthy mitochondria in the mother's fertilised egg cell can be 'carried over' to the mitochondrial donor's fertilised egg cell, along with the mother's nuclear material. A related concern is that that even if the proportion of unhealthy mitochondria (owing to this 'carryover') is initially very low in the resulting embryo, this proportion can subsequently increase in the cells of the developing embryo/fetus during pregnancy, a phenomenon sometimes called 'reversion' or 'reversal'.

The two research papers indicate that neither carryover nor reversion posed a significant problem in any of the eight children born following PNT. However, the children will continue to be monitored as a precaution.

At birth, levels of unhealthy mitochondria were so low (less than three percent) in five of the eight children that the unhealthy mitochondria could only be detected, if at all, by 'next-generation' sequencing of DNA. In the remaining three newborns, the highest proportion of unhealthy mitochondria detected was 16 percent in one child's blood and 20 percent in the same child's urine. This is well below the 80 percent threshold that must usually be reached, in order for there to be any clinical symptoms of mitochondrial disease.

In the case of another child, the proportion of unhealthy mitochondria at birth was five percent in blood and nine percent in urine. This had fallen to undetectable levels by the time the child was 18 months old.

Professor Mary Herbert, lead author of the embryology paper and coauthor of the clinical outcomes paper, said: 'Mitochondrial donation technologies are currently regarded as risk reduction treatments owing to the carryover of maternal mitochondrial DNA during the mitochondrial donation procedure. Our ongoing research seeks to bridge the gap between risk reduction and prevention of mitochondrial DNA disease by addressing this problem.'

How did we get here?

The UK was the first country to legislate specifically to permit the use of mitochondrial donation. A new law was passed in 2015 (see BioNews 711, 744, 764, 770, 785, 792 and 826) after wide-ranging public campaigns, discussions and consultations (see BioNews 605, 661, 662, 673, 678, 682, 684, 698, 771 and 788).

PET (the Progress Educational Trust, the charity that publishes BioNews) played a leading role in bringing about the change in the law, organising high-profile public events about the issue (see BioNews 676). One of these PET events was held at the Houses of Parliament (see BioNews 789a) and was referred to in the House of Commons the following day, during a debate which concluded with MPs voting overwhelmingly in favour of permitting mitochondrial donation (see BioNews 789b). Other UK charities that played a key role in campaigning for the law to be changed include the Lily Foundation, Genetic Alliance UK and Wellcome.

The change in law was also informed by several assessments of the safety and efficacy of mitochondrial donation, conducted by an expert panel convened by the HFEA (see BioNews 599, 605 and 757). After the law was changed, this expert panel was reconvened to conduct further assessment, before the HFEA was prepared to consider licensing the use of mitochondrial donation in treatment.

Following the final assessment by the expert panel, clinics were able apply to the HFEA for a licence permitting them to offer mitochondrial donation treatment, and the Newcastle team was granted such a licence in 2017 (see BioNews 893). At present, clinics are also required to seek HFEA approval for every woman whom they wish to treat on a case-by-case basis, and the first such approval was granted to the Newcastle team in 2018 (see BioNews 936).

Meanwhile, babies with donated mitochondria have been born in other countries, where mitochondrial donation is not prohibited by established law or where the legal situation is ambiguous (see BioNews 871, 885, 895 and 995). The first confirmation that babies had been born following mitochondrial donation in the UK came in 2023, in response to a freedom of information request (see BioNews 1189a and 1189b).

Sarah Norcross, director of PET, issued the following response to the publication of the new research papers by the Newcastle team: 'We could not be more delighted by the news that eight babies with donated mitochondria have been born in the UK, and that all of these children have made normal developmental progress. The medical and scientific work at Newcastle, and the policy and legal work that preceded it, have set a high standard for introducing new reproductive technology in a careful and scrupulously regulated way.'

The Lily Foundation – which helps patients and families affected by mitochondrial disease, and which funds related research – was founded by Liz Curtis, whose eight-month-old daughter Lily died of mitochondrial disease in 2007. Curtis said: 'We fought long and hard for this change so that families could have choices. After years of waiting, we now know that eight babies have been born using this technique, all showing no signs of mito. For many affected families, it’s the first real hope of breaking the cycle of this inherited condition.'

Mitochondrial disease

Mitochondria are tiny structures that exist in almost all of the cells in the human body. They house the cellular machinery that converts the energy from our broken-down food into a specific molecule, that provides energy for everything that our cells, tissues and organs do.

When mitochondria do not function properly, this can give rise to a wide range of symptoms, some of which are severe and can result in death during infancy. There is no cure for such mitochondrial diseases. Professor Sir Doug Turnbull, emeritus professor of neurology at Newcastle University – lead author of the new clinical outcomes paper, and coauthor of the embryology paper – noted at a press briefing to announce the papers that there has been no success in developing cures since he began research in this field during the 1990s.

Mitochondrial diseases are genetic conditions, caused by mutations in genes that encode vital proteins. Some of the genes that cause mitochondrial diseases are found within the chromosomes of the cell nucleus, but mitochondria also contain their own DNA in the form of a circular chromosome. There are multiple copies of this circular chromosome within each individual mitochondrion, and there can be hundreds or even thousands of mitochondria within a human cell. The mitochondrial DNA (mtDNA) within the circular chromosome encodes 13 proteins, which are essential for the functioning of the mitochondria.

Whereas nuclear DNA is inherited from both parents, mtDNA is naturally inherited from the mother except in very rare cases (see BioNews 978). Consequently, if a woman carries disease-causing mtDNA mutations, then she is liable to transmit these mutations to her children. For some women, this means that the only way to have a genetically-related child is via mitochondrial donation, in which the mother's nuclear material is combined with mitochondria from a donor.

Nick Meade, chief executive of Genetic Alliance UK, offered the following response to the new papers from the Newcastle team: 'Most rare conditions do not yet have a cure or treatment, so for families affected, reproductive choice techniques are the only opportunities to take control of the impact of the condition.'

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Mitochondrial donation pioneers Professor Mary Herbert and Professor Sir Doug Turnbull will discuss their work at the free-to-attend online event Mitochondrial Donation: Does It Work? What Next?, taking place on Wednesday 8 October 2025.

This will be a joint UK/Australian event. Find out more and register here.