The "chemical chatter" that determines whether an embryo can implant in the womb has been detailed by scientists.
It is important for the lining of the womb to know if an embryo is healthy before allowing it to implant, but how this is done was unclear.
A study by the universities of Southampton and Warwick has shown the amount of a chemical, trypsin, helps determine whether an embryo implants.
It is hoped the discovery will lead to new fertility treatments.
When a sperm fertilises an egg and begins to divide and grow, the process does not always go smoothly.
"A lot of embryos have chromosomal abnormalities so there must be a mechanism to stop a pregnancy," fertility scientist Prof Nick Macklon told the BBC.
"So the question is how does the endometrium [the lining of the uterus] detect the quality?"
Research to be presented at the Reproductive Biology conference at the University of Southampton shows chemicals given off by an early-stage embryo give the endometrium clues.
Trypsin seem to be critical. If the chemical is at the right level, then it changes the nature of womb to make it more inclined to accept the embryo. But if the levels are out, then the lining becomes stressed and less likely to accept.
Prof Macklon said: "With a good embryo then the good response is turned up to allow implantation, if there's a bad embryo then the endometrium responds to reject the embryo.
"We've identified the pathways which signal this change and this has a big clinical context as implant failure is still the major cause of IVF failure."
He said large numbers of couple had fertility problems and that he hoped breakthroughs in understanding how an embryo implanted successfully could improve the odds in IVF.
Monitoring the signals given off by an embryo could help choose which ones to implant during IVF, although this idea has not yet been tested.
It could also help to explain recurrent miscarriage, which in some cases has been linked to women being "super-fertile" and accepting embryos that should be rejected.
However, research will have to progress carefully as forcing a poor quality embryo to implant would lead to serious problems.
Prof Siobhan Quenby, who is also a spokeswoman for the Royal College of Obstetricians and Gynaecologists, said: "It's a really exciting mechanism, but the challenge is turning it into a treatment, I lie in bed at night thinking about how you would turn this into a treatment.
"It's beautiful science, but we're not there yet."