Birth control pills (oral contraceptives) contain progestin with or without estrogen. They prevent pregnancy by stopping ovulation (the release of eggs by the ovaries) or by thickening mucus in the cervix (so that sperm cannot pass through the cervix into the uterus).
Oral contraceptive dispensers contain a 28-day supply. Most contain 21 active pills and 7 inactive pills. Dispensers with progestin-only pills contain 28 active pills.