The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles. Both LH and FSH attain a peak level in the middle of the cycle (about 14th day). Rapid secretion of LH leading to its maximum level during mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). Concentration of estrogen in blood is also high around 14th day and reaches its peak.