Oral contraceptives are medicines taken by mouth to help prevent pregnancy. Also known as birth control pills, they contain artificially made forms of two hormones produced naturally in the body. These hormones, estrogen and progestin, regulate a woman's menstrual cycle. When taken in the proper amounts, following a specific schedule, oral contraceptives are very effective in preventing pregnancy.

Oral contraceptives have several effects that help prevent pregnancy. For pregnancy to occur, an egg must ripen inside a woman's ovary, be released, and travel to the fallopian tube (the passageway from the ovary to the uterus). A man's sperm must also reach the fallopian tube, where it fertilizes the egg. Then the fertilized egg must travel to the woman's uterus (womb), where it lodges in the uterus lining and develops into a fetus. The main way that oral contraceptives prevent pregnancy is by keeping an egg from...