Human embryos have been kept alive in a petri dish for an unprecedented 13 days, allowing scientists to finally see what happens in the mysterious days after implantation in the womb.
Cambridge University have created a special thick soup of nutrients which mimics the conditions in the womb, allowing the embryo to attach, and begin dividing into groups of cells which will eventually form the foetus, placenta and yolk sac.
Previously an embryo had to implant in the womb by day seven to survive, but it is impossible to see what is happening inside the mother at this stage, so scientists were in the dark about the cellular and molecular changes taking place.
Crucially it is during that period that two thirds of pregnancies fail because the embryo does not implant properly.
Now that scientists can see the steps needed for healthy embryo development it will help them understand why things go wrong, potentially improving IVF rates.
"It is a most enigmatic and mysterious period of our development which we have never had any access to," said lead author Professor Magdalena Zernicka-Goetz of the University of Cambridge.
"Implantation is a milestone in human development as it is from this stage onwards that the embryo really begins to take shape and the overall body plan are decided
"It is also the stage of pregnancy at which many developmental defects can become acquired. But until now, it has been impossible to study this in human embryos.
"This new technique provides us with a unique opportunity to get a deeper understanding of our own development during these crucial stages and help us understand what happens, for example, during miscarriage. It was incredible, it was the happiest day of my life when we saw this."
Although studies are just beginning the scientists have already made new findings including the fact that embryos create two cavities to grow in, one for the foetus and one for the placenta.
Dr Simon Fishel, founder and President of Care Fertility Group, adds: "This is about much more than just understanding the biology of implantation embryo development.
"Knowledge of these processes could help improve the chances of success of IVF, of which only around one in four attempts are successful."
Currently UK law bans laboratories for growing embryos for longer than 14 days as after two weeks, twins can no longer form, and so it is deemed that an individual has started to develop.
The time limit has remained unchallenged while scientists were unable to keep embryos alive beyond seven days, but the new breakthrough could lead to calls for the threshold to be extended.
Professor Zernicka-Goetz said it would be useful to be allowed to follow the development of embryos for a 'a few more days' after the current cut off period.
The Nuffield Council on Bioethics is also planning a meeting on the possibility of changing the limit.
Professor Robin Lovell-Badge, Group Leader, The Francis Crick Institute, said: "Proposing to extend the 14-day limit might be opening a can of worms, but would it lead to Pandora's box, or a treasure chest of valuable information ?
"This is not a question to be left to scientists alone."
The embryos used in the research were donated by couples undergoing fertility treatment.
The research was published in the journal Nature.