For the first time, scientists have successfully grown two-week-old human embryos in the lab so they could study their development.

Scientists have for the first time grown human embryos outside of the mother for almost two full weeks into development, giving insight into what they say is the most mysterious stage of early human life.

Scientists had previously only been able to study human embryos as a culture in a lab dish until the seventh day of development when they had to implant them into the mother's uterus to survive and develop further.

But using a culture method previously tested to grow mouse embryos outside of a mother, the teams were able to conduct almost hour by hour observations of human embryo development to see how they develop up to day 13.

"This is the most enigmatic and mysterious stage of human development," said Magdalena Zernicka-Goetz, a University of Cambridge professor who co-led the work. "It is a time when the basic body shape is determined."

The work, covered in two studies published on Wednesday in the journal Nature and Nature Cell Biology, showed how the cells that will eventually form the human body self-organise into the basic structure of a post-implantation human embryo.

"Embryo development is an extremely complex process and while our system may not be able to fully reproduce every aspect of this process, it has allowed us to reveal a remarkable self-organising capacity ... that was previously unknown," said Marta Shahbazi, a researcher at Britain's University of Cambridge who was part of the research teams.

Robin Lovell-Badge, an expert in stem cells at Britain's Francis Crick Institute who was not directly involved in this work, said it provided "a first glimpse" of how the early human embryo develops at the point when it would usually implant in the womb lining, becoming invisible and impossible to study.

As well as advancing human biology expertise, the knowledge gained from studying these developments should help to improve in-vitro fertilisation (IVF) treatments and further progress in the field of regenerative medicine, the researchers said.

But it also raises the issue of an international law banning scientists from developing human embryos beyond 14 days, and suggests this limit may have to be reviewed.

Zernicka-Goetz, who spoke to reporters in London, said a wealth of new information could be discovered if human embryos could be grown in a lab dish for just a few days more.

"Longer cultures could provide absolutely critical information for basic human biology," she said. "But this would of course raise the next question - of where we should put the next limit."

Sarah Norcross, director of the Progress Educational Trust, a charity which campaigns for people affected by infertility and genetic conditions, agreed that the research raised questions around the 14-day limit and said the international scientific community should "decide whether it is necessary and desirable" to extend it.

"A public discussion of the rights and wrongs of this would need to follow before any change in law could be contemplated," she told Reuters.