Scientists hope the discovery will prove to be a milestone in the development of new pregnancy treatments.
An early stage embryo can only develop into a foetus and baby if it successfully implants into the soft lining of the womb.
Implantation failure is a major cause of miscarriages, yet many aspects of it remain a mystery.
Researchers from the University of Warwick found high quality embryos secrete a chemical called trypsin that produces a reaction in the womb lining, rendering it supportive of implantation.
In low quality embryos with a high proportion of abnormal cells, the chemical's absence triggers an alarm response in the womb which results in rejection.