An adequate dialogue between embryo and endometrium should exist in the implantation.
Embryo implantation takes place when the embryo in blastocyst stage adheres to maternal endometrium. This happens at some point called implantation window and it takes place about six or seven days after fertilisation. The change from non-receptive to receptive endometrium only happens if there is an appropriate hormonal environment and if the blastocyst express adequate molecules to induce the change (selectins, cytokines, etc.).
In order that this adhesion occurs, there must be an appropriate uterine environment and an appropriate dialogue between the embryo and the endometrium. Different studies on embryo implantation have shown which role different molecules play in this proces such as cytokines, integrins, adhesion molecules and growth factors.