Oral contraceptives (birth-control pills) are used to prevent pregnancy. Estrogen and progestin are two female sex hormones. Combinations of estrogen and progestin work by preventing the release of eggs from the ovaries (ovulation) and changing the cervical mucus and the lining of the uterus. Oral contraceptives are a very effective method of birth control, but they do not prevent the spread of AIDS and other sexually transmitted diseases.