A small sample of the lining of the uterus (endometrium) is obtained in the latter part of the cycle to see if the endometrium has properly matured, under the influence of the hormone Progesterone. If there is a "lag" e.g. the endometrium has not reached the proper stage, there exists a condition known as a Luteal Phase Defect, which may be treated hormonally. Some programs do this prior to an IVF cycle.