The endometrium is the inside layer of the uterus where the embryo embeds and continues its development. The uterus must be free of obstructions such as fibroids or significant congenital abnormalities. Endometrial cells have the capacity to rapidly grow and develop under the influence of the hormones estrogen and progesterone. This “growth” results in the increased vascularity and endometrial width which is needed to support a developing embryo.

A small sample of the endometrium is taken during the luteal phase of the ovulatory cycle approximately twelve days after the LH surge. The biopsy is examined to see if the cellular development of the endometrium “matches” what is predicted for day the sample was taken. If endometrial cellular development matches the biopsy, the biopsy is termed “in phase”. If the biopsy shows less than predicted development for that cycle day, the biopsy is said to be “out of phase” and a luteal phase defect may exist.

Progesterone is administered in the next ovulatory cycle to correct the luteal phase defect.