Scientists say a breakthrough in growing embryos will improve fertility treatments and revolutionise knowledge of the earliest steps to human life. 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 published in the journal 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.

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.