As the zygote traverses the maternal tubal and uterine cavity, it becomes a morula (a cluster of divided cells of the fertilized ovum) and matures into a blastocyst. The blastocyst then looses its zona pellucida (a clear protective layer that facilitates transport as it is nonadhesive), and develops an inner cell mass and an outer protective layer, the trophectoderm. Implantation is the term applied to the process by which the blastocyst attaches to the wall of the uterus and begins to send fingers of tissue (chorionic villi) into the wall of the uterus as anchors.

Blastocyst implantation and successful establishment of pregnancy require delicate interactions between the embryo and the maternal environment. A highly-coordinated process is set into motion whereby specialized cells of the embryo, the trophectoderm and the throphoblast, establish contact with the specialized maternal tissue, the uterine endometrium. Uterine implantation...