The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium (wall of the uterus) . Such an endometrium is necessary for implantation of the fertilized ovum. In the absence of fertilization, the corpus luteum degenerates resulting in decline in the progesterone level. This causes disintegration of the endometrium leading to menstruation, making a new cycle. Estrogen also helps in the regulation of menstrual cycle by controlling the growth of the uterine lining during the first part of the cycle.