The major events in menstrual cycle include menstruation, the follicular phase and the luteal phase, after which the next cycle begins. Both LH and FSH attain a peak level in the middle of the cycle (about the 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle, called LH surge, induces rupture of Graafian follicle and thereby the release of ovum (ovulation). The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy.