In July 1978, Louise Brown was born in Manchester, UK—the first baby ever to be conceived by in vitro fertilization (IVF). This made headlines around the world, and soon sparked debate about the ethics of IVF and the rapid pace of development in this and related technologies. In 1982, in response to public concern, the UK government set up a Committee of Inquiry into Human Fertilisation and Embryology to investigate the situation and make some recommendations.

In 1984, the committee produced the Warnock Report, which recommended (amongst other things) that human embryos could be cultured in vitro for no longer than 14 days. This was eventually implemented in legislation in the UK’s Human Fertilisation and Embryology Act 1990. This firm limit to scientific research on embryos helped to alleviate public concern, and researchers were satisfied because the 14-day limit was far beyond what was actually possible.

The 14-day rule was widely adopted around the world, and for over 30 years has had few critics.

However, in 2016 it became possible to culture human embryos for longer than 14 days, and unsurprisingly, some researchers are now pushing to have the limit extended, so as not to hamper future research. Of course, claims of significant benefits of such research are made. An extension will apparently help to enable rates of miscarriage to be reduced, IVF outcomes to improve, and the safety of new techniques such as gene editing to be tested.

Gradually, the chorus of voices advocating for an extension is increasing. It’s an understandable trend—because we can already experiment on embryos for up to 14 days, many people may see little harm in extending this to 28 days or more. What difference could an extra 14 days make? The embryo lives a little longer, that’s all.

There are, however, numerous reasons why an extension is not a good idea.

The first reason is that any experimentation on human embryos is controversial, and for many, immoral. The Warnock Committee itself had several members that held this view—that human embryos are human beings that, in time and in their natural environment, will likely develop into people just like us. If it is wrong to experiment on us, then because we were once embryos, it would have been wrong to experiment on us then. The Warnock Committee came up with the 14-day rule as a compromise — they had to reconcile a wide range of incompatible views about the value of human embryos. Inevitably, perhaps, the majority on the committee succumbed to the promise of scientific progress and the possibility of helping infertile couples have children of their own.

Of course, the 14-day rule itself had to come from somewhere—the Warnock committee needed a plausible justification for it.

They came up with an argument about the importance of our individual identity. It was known that up to 15 days or so, embryos could split, resulting in twins. The report argued that this meant the embryo was not a unique individual until twinning was no longer possible, and therefore it can be used and disposed of prior to this point. This is quite a dubious claim. Most embryos never twin, and so in most cases they seem to be the same individual from fertilisation. Even if twinning does occur, we don’t know what happens to the original embryo. It might still be there as one of the two resulting embryos; it may not be. In fact, there is a lot we don’t understand about twinning.

It’s even possible that twinning occurs much earlier than 14 days—for example, Gonzalo Herranz has proposed a different model of twinning to the traditional model, in which it happens at the first division of the zygote. This is very soon after fertilisation. We don’t have definitive evidence for Herranz’s model or the traditional model, and if Herranz’s model proves to be more accurate, then the Warnock Committee’s reasoning for a 14-day limit is completely unjustified (even though it is severely flawed under the traditional model).

This is unlikely to make any difference, because the Warnock Committee’s conclusion was a fudge designed to satisfy public concern and allow researchers latitude to experiment on embryos. The twinning argument is a convenient reason to settle on a limit that limits dissent to the minority who believe that human embryos are valuable human beings. If the consensus in the future is that this reasoning is no longer valid, it seems very unlikely the 14-day rule would be removed. It has been with us for decades, widely implemented in legislation, and the outcry in the research community would be immense. Some other justification would no doubt be produced—the benefits of experimentation for us all, perhaps, or some other reason why embryos are not as valuable as other human beings.

So, how do we decide if the 14-day rule should be extended to a 28-day rule? Clearly, if human embryos are considered valuable human beings, it shouldn’t be—the 14-day rule itself should be removed, not extended.

However, even on the Warnock Committee’s flawed reasoning, there isn’t a good case for extending the rule to this extent. If the committee’s argument regarding twinning is accepted, then 28 days is well beyond the twinning limit, which in very rare cases is now thought to be up to 21 days (on the traditional model). As I have noted, though, the traditional model has been challenged by Herrantz’s early twinning model, and this should be taken into consideration.

An alternative argument employed by Elsejin Kingma and cited by others is that once a research embryo can no longer be successfully implanted in a uterus, it has lost its potential to become a valuable human being. So, according to Kingma, a 14-day embryo no longer has potential to do so, and obviously a 28-day embryo does not either.

However, this reasoning would also justify experimenting on embryos and fetuses of up to 20 weeks or more, as they also do not have the potential to become valuable human beings (as they are still not viable at this point). It seems likely most people would find this objectionable, and so it casts doubt on Kingma’s reasoning.

An additional problem that Kingma herself points out is that if artificial womb technology is perfected, then all embryos and fetuses will have the potential to become valuable human beings—implying that research on all embryos will have to be halted once this occurs, even those less than 14 days old. This might sound speculative (and it is), but goat fetuses have already been successfully gestated for the last 4 weeks of pregnancy in what has been called a “biobag”.

So, if we evaluate proposals to extend the 14-day rule by the Warnock committee’s reasoning, there are few reasons to agree to extend. The twinning principle the committee used implies that it could be extended by a few days at most, not 14 days, and it is possible that this justification could be undermined as research into twinning continues. From a pragmatic point of view, researchers have not come close to exploiting the 14-day limit—most discoveries have come from research into the first 7 days.

Also, since implantation usually occurs in the first few days, it isn’t clear how accurately a post-implantation embryo culture will mimic the in-utero environment, particularly as time goes on. It might be that the applicability of further research is of limited value, rather than producing the benefits that those in favour of an extension are fond of touting.

To summarise, there are no compelling reasons to extend the 14-day limit for now, even if the embryo is not considered to be a valuable human being. Doing so might work against the aim of the Warnock committee, which was to alleviate public mistrust of science. Critics have always warned about the dangers of a slippery slope and extending the limit might confirm those fears.

Interestingly, even Mary Warnock, the chair of the Warnock committee and its namesake, quite recently stated that the 14-day rule should stay in place for now. At the very least, an extensive public debate is required prior to any decision being made.

Bruce Blackshaw

Bruce P Blackshaw is a doctoral researcher in the Department of Philosophy at the University of Birmingham.