Birth control

Highly effective contraceptive methods involve the use of hormone drugs in order to prevent pregnancy. Oral birth control pills contain the sex hormones estrogen and progesterone. These drugs, also called oral contraceptives, hinder both the normal release of an egg once a month and the attachment of an egg to the uterus. Birth control pills are relatively expensive, require regular use to prevent pregnancy, and may produce harmful side effects in some women.

Excerpt from the "Birth control" article, The World Book Encyclopedia © 1999