**1. Background**

In order for pregnancy to occur, two events must take place:


Following fertilization, the developing embryo must embed itself within the endometrium. In order for this to take place, both the embryo and uterus require the secretion and suppression of specific proteins that allow for implantation, including the expression of adhesion molecules on the cell surface, secretion of growth factors, and morphologic cell differentiation. Similarly, the embryo must also have developed to the blastocyst stage and be able to secrete appropriate protein factors for invasion and immunosuppression. These events between the embryo and endometrium must occur concomitantly in order for proper implantation to occur. If either the embryo or endometrium is asynchronous to the other, implantation will not take place, inducing the next cycle of menses. While great scientific advances have been made in the field of assisted reproduction since its inception in 1978, it is only within the last

10 years that we have really begun to understand this intricate network of synchronized events that allows the embryo and uterus to meet and become one.
