In 1984 a line was drawn in the sand; research on human embryos was limited to 14 days of development, effectively setting the point at which an embryo becomes too human to be experimented upon. Today, however, recent scientific advances have led to calls by some researchers for the rule to be relaxed.

The constraint on the experimentation on embryos to 14 days post-fertilization, the so called “14-day rule”, was first proposed in 1979 by the US ethics advisory board. Currently 12 countries around the world have enshrined the limit into law, while five others have 14 days written in their scientific guidelines. So there exists a solid international consensus, many nations and scientific advisory bodies have come together to agree on this cut-off, but why? What is so special about 14 days?

The first reason is a philosophical distinction; fourteen days old is the first point at which an embryo can be said to have an individual identity, as before this the embryos can still fuse and split to form monozygotic twins. Secondly, at this point a band of cells known as the primitive streak is formed, marking the beginnings of a head-to-toe axis. This is the beginning of the transition from a ball of cells towards something that begins to resemble a foetus. Researchers also like it because it’s easy to identify, providing a useful marker of this point in development since this might not always happen at the same speed.

Until recently the rule was not a concern for scientists. Rarely were researchers able to maintain embryos past 7 days outside of the womb. This changed last year when two ground-breaking studies reported new techniques to create a chemical mimic of the womb.  Using these methods they could grow the embryos for as long as 13 days, and for the first time found themselves butting up against the 14-day rule.

Now some scientists are calling on the U.K. government to look again at the rules, and extend the limit past 14 days. Once such advocate for change is Professor Simon Fishel, who was part of the team involved in the creation of the first IVF baby. He has proposed moving the limit to 28 days, speaking with the BBC he said:

“I believe the benefits we will gain by eventually moving forwards when the case is proven will be of enormous importance to human health”. In particular, many cite the impact that such work would have on our understanding and ability to treat fertility issues.

But there are some that are opposed to an extension to the limit, Professor Fishel concedes: “There are some religious groups that are fundamentally against IVF, let alone IVF research in any circumstances, and we have to respect their views.”

Indeed, some fear the extension of the 14-day rule could be the beginning of a slippery slope. Bioethicist and founder of the Centre of Bioethics and Emerging Technologies David Jones is one critic of the idea of an extension, he told the BBC:

“It would be a stepping stone to the culturing of embryos and even foetuses outside the womb. You are really beyond the stage when the embryo would otherwise implant and that is a step towards to creating womb like environment outside. People will then ask why can’t we shift it beyond 28 days.”

How then should we reconcile people’s ethical concerns with a need to update the rules on embryo research to the 21st century? One challenge will be reaching an international consensus on where a new line should be drawn.

The question still remains; whose responsibility is it to define the limits on human embryo research? Is it scientists and researchers or bioethicists and theologians?

This is ultimately a question for us all, for society as a whole. It is our responsibility for us to consider the views of these different groups, to weigh the benefits to society versus the moral considerations, and inevitably to ask ourselves where we feel comfortable setting the limit.


  1. Deglincerti,A. et al.Nature http://dx.doi. org/10.1038/nature17948 (2016).
  2. Shahbazi, M. N. et al. Nature Cell Biol. http://dx.doi. org/10.1038/ncb3347 (2016).

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