The control mechanisms involved in implantation can be divided into those operating from outside the uterus and intracellular controls within the organ. The hormones of the ovary ensure that uterine preparation is synchronized with the presence of a mature blastocyst in the uterine lumen, whilst the intracellular controls regulate and integrate the changes which take place within the organ and between it and the blastocyst. A prominent feature of these changes in many species is the transformation of the connective tissue stromal cells into specialized decidual cells in which the blastocyst comes to lie, either by passing through the uterine epithelium or by degeneration of the epithelium around it. This transformation will be referred to as the decidual cell reaction (DCR).

In the majority of animals implantation takes