Contraceptive implants are soft capsules, about 1½ inch long, under the skin in a woman’s upper, inner arm. The capsules constantly give off tiny amounts of an artificial hormone called progestin. This prevents pregnancy by thickening the cervical mucus so that sperm can’t get into the uterus and by stopping ovulation.
There are different types of contraceptive implants being used around the world. The one used in the United States is called Implanon, and is a single rod that lasts three years. It can be used for less than three years because it can be taken out at any time.
In all of the studies so far, Implanon has been 99% effective. In the study that was done in the United States to test Implanon’s effectiveness, no one got Implanon who weighed more than a certain amount for a given height. If a woman weighs more than the weight on this chart, Implanon is probably over 99% effective, just as it is for other women, but it hasn’t been fully studied.