The third parent

The news this month that eight healthy babies have been born following IVF using genetic material from three people has been universally welcomed as a breakthrough in the prevention of mitochondrial disease (see BioNews 1298a and 1298b). But in 2015, when Britain became the first country to authorise the treatment, the so-called 'three-parent baby' was the source of intense controversy.

On one side were those who celebrated a national 'first', just as happened when Louise Brown, the first ever 'test-tube baby', was born in Oldham in 1978 (see BioNews 960, 961 984 and 996). As with IVF more broadly, the possibility of becoming parents of healthy children for people who otherwise couldn't is something to be welcomed.

On the other side, the cry went up that this was a slippery slope, the thin end of the wedge, or any other cliché that you care to click on (see BioNews 747 and 790). At any rate, it is not a happy event; it would take us one step further towards the spectre of 'designer babies'. The issue here was about meddling with nature for the sake of hubristic human desires: seeking to ensure the production of a 'perfect' baby, and in the process creating a new monstrosity of triple parenthood.

The negative line about the new baby-making procedure was most often countered in these arguments by pointing out how negligible, in reality, the contribution of the shadow third party is. All that happens is that a dysfunctional aspect of the (principal) mother's DNA is, as it were, overwritten by the provision of healthy mitochondrial matter from the donor. This means that the resulting baby will not go on to develop debilitating hereditary diseases that might otherwise have been passed on. The handy household metaphor of 'changing a battery' was much in use for this positive line: the process should be seen as just an everyday update.

The way the argument was made – presenting the new input as a simple enabling device – revealed a need to switch off any suggestion that this hypothetical extra parent has any fundamental importance. The real identity of the future child must be seen to come from a primary mother and father, and them alone.

But in actuality, three-parent babies have been around for some time. According to the established facts of life, it seems obvious enough that a baby has two biological parents, and that the contribution is distributed between them with the god-given genetic equality of exactly 23 chromosomes each. But that is to forget a further biological element, the female body in which the new baby grows from conception (or just after, in the case of IVF) to birth.

This used to be elided in the obvious certainty of maternal identity. If a woman gave birth, then the baby was 'biologically' hers in every way: she was pregnant, the ovum had come from her. Whereas a father, until DNA testing, was always uncertain, in the sense that there was no bodily evidence of any particular man having engendered any particular child: pater semper incertus est ('the father is always uncertain'), in the legal phrase.

But modern medical science has brought about the practical as well as the theoretical separation of egg and womb, breaking up what had previously appeared as – and still usually is – one single biological mother. With this not uncommon doubling of motherhood through IVF, three-parent babies have been with us for quite some time.

Twofold motherhood plays out in various possible scenarios. Conception outside the womb – in other words, in vitro fertilisation – leads to the possibility of a woman giving birth to a baby of which she is not the genetic mother, because the embryo transferred to in a womb doesn't have to be formed from an egg taken from the same woman (see BioNews 1236, 1238 and 1273). Depending on whether that baby is going to become, postnatally, the child of the birth mother or else of the woman who provided the egg, this pregnant woman will be referred to as either the mother or the surrogate (see BioNews 1187, 1188, 1203 and 1299).

Or, to tell the same story from the genetic point of view: IVF leads to the possibility of one woman's egg making an embryo which develops in another woman's womb. Depending on which of them is to raise the child, she will be regarded either as the egg donor (for the mother, who becomes pregnant), or as the mother (who used a surrogate).

In all these scenarios, biological motherhood is already divided between two women, with one or other element – the egg or the pregnancy – being emphasised, depending on which of them is to become, in future, the baby's mother.

Depending on whose experience is being described, the drive is always towards the subordination of one biological contribution so as to validate the other (and also to remove the risk that the discounted proto-parent, the 'donor' or the 'surrogate', might have any postnatal responsibilities or rights). This is what happened again with the controversy over the additional mitochondrial 'parent'. But biologically, three-parent babies were nothing new.

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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.