For the first time, embryos have been grown past the point they would normally implant in the womb.

The research, in the UK and US, was halted just before the embryos reached the legal limit of 14-days old.

But in an ethically-charged move, some scientists have already called for the 14-day limit to be changed.

The earliest steps towards human life are largely a mystery, but the research in Nature and Nature Cell Biology, has been able to study embryos for longer than ever before.

About a week used to be the limit - with scientists able to grow a fertilised egg up to the stage it would normally implant into the womb.

The blastocyst (bottom right) was the old limit of embryo development in the lab.

But they have now found a way to chemically mimic the womb to allow an embryo to continue developing until the two week stage.

It requires a combination of a nutrient-rich medium and a structure the embryo can pretend to "implant" upon.

The experiments were deliberately ended at the 13-day stage - just before the legal limit, but far beyond anything that has been achieved before.

The research is already providing insight into how an embryo starts the process of organising itself into a human being.

It is a crucial time when many embryos acquire developmental defects or fail to implant.

For example the study has allowed scientists to see the formation of the epiblast in 10-day-old embryos.

It is a crucial and tiny cluster of cells that eventually forms the human being, while the cells surrounding it go on to make the placenta and yolk sac.

The epiblast in purple on the left and white-purple on the right developing inside the embryo.

Prof Magdalena Zernicka Goetz, from the University of Cambridge, said she "couldn`t remember being more happy" than succeeding in growing the embryos.

She told the BBC: "It actually allows us to understand the very first steps in our development at the time of implantation where the embryo, really for the first time, reorganises itself to form the future body.

"Those steps we didn`t know before so it has enormous implication for reproductive technologies."

Ethical conundrum

There is international agreement that experiments should not allow embryos to develop past 14 days.

This research is pushing right up against the legal limits and some scientists are already making the case for the 14-day limit to be reviewed.

Prof Azim Surani, from the Gurdon Institute, said: "In my opinion, there has been a case to allow culture beyond 14 days even before these papers appeared. "

The 14-day limit is decades old and is thought to represent the first point at which an embryo becomes an "individual" as it can no longer form a twin.

But Prof Daniel Brison, from the University of Manchester, argued: "Given the potential benefits of new research in infertility, improving assisted conception methods, and in early miscarriage and disorders of pregnancy, there may be a case in the future to reconsider this."

The Nuffield Council of Bioethics - which played a prominent role in the debate around three-person babies - has announced it will review the issue.

It is an area that could spark huge ethical debate in the coming years.

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