The regulation of synchrony between embryo and endometrium has not been fully addressed clinically in the human. This limitation affects IVF significantly in that up to 30% of embryo transfers may be affected by endometrial receptivity factors. The endometrium is a highly specialized, hormonally regulated tissue which is only receptive to embryo communication/interaction for a self-limiting period referred to as the window of implantation. Currently, there is no way to non-invasively determine the time of optimal receptivity, with IVF transfers performed on the assumption that the WOI was sufficiently wide enough to accommodate implantation in most patients and most treatments. During this period, the endometrial epithelium acquires optimal receptivity by progressing through specific structural, functional, and morphological changes induced by pre-ovulatory estrogen and progesterone and post-ovulatory progesterone, LH, and hCG.