Norplant is a long-acting hormone that is inserted under the skin and prevents conception for up to five years. Depo-Provera is also a hormone, but it is injected into a muscle and provides protection against pregnancy for three months. The hormone in Norplant and Depro-Provera is progestin, a synthetic hormone similar to one found naturally in a woman's body.

Both hormones are about 99% effective in preventing pregnancy. Neither of these methods provide protection from AIDS or other sexually transmitted diseases.

Given as a shot, Depo-Provera works in several ways to prevent conception. First, it prevents the egg (ovum) from maturing and being released, and it causes the mucous in the cervix (opening into the uterus or womb) to thicken, making it difficult for the sperm to enter. Depo-Provera also causes the lining of the uterus to become thinner, making implantation of a fertilized egg unlikely. The...