Estradiol is one of the main components of naturally occurring estrogens and is the major estrogen secreted during the menstrual cycle. The serum levels of estradiol are low during the follicular phase rising gradually until about one day before ovulation when a marked rise in the estradiol level occurs (Ovulatory Peak). The estradiol level falls rapidly at, or right after ovulation and is again within the levels of the follicular phase. There is a second rise of estradiol around day 21 of the cycle (Luteal Peak). The levels then decline gradually to the lowest level at the onset of the next menstrual cycle.