Oral contraceptives are one of the most effective forms of birth control .The most widely used type of oral contraceptive in the United States today is the combination preparation of an (ethinyl estradiol, mestranol) and a progestin (norgestrel, levonorgestrel) .

Combined Oral Contraceptive Pill mechanism of action, or “How the pill works”

Mechanism of action of oral contraceptives or "How the pill works"

This animation depicts inhibition of ovulation as the primary mechanism of the contraceptive action of sequential and combination birth control preparations. Ovulation is prevented by the suppression of the midcycle surge of FSH and LH.

Estrogens are most active in inhibiting FSH release, but at high enough doses, they also inhibit LH release. In low-dose combination products, the progestin causes LH suppression.The progestin component is also important in causing withdrawal bleeding at the end of the cycle.