The endometrial tissue on the inside of the uterus undergoes changes in response to the hormones released during the ovulatory cycle.  The changes include both thickening of the layer, and morphologic changes when viewed under the microscope.  The morphologic changes have been classified, and a pathologist is able to tell what cycle day a woman is relative to her ovulation from a small tissue sample or a biopsy.  The endometrial biopsy is best performed at the time which is midpoint between ovulation and her expected menses.  The small amount of tissue is obtained using an endometrial biopsy pipette, and isn’t much more uncomfortable than a pelvic exam.  Simultaneous with the biopsy blood is drawn for a progesterone determination, and the physician is able to conclude that ovulation has occurred based upon the results.