Birth control implants are devices that are inserted under a woman's skin. They release a hormone that prevents pregnancy.
Two similar implants available in the U.S. are Implanon and Nexplanon. However, Implanon is gradually being replaced by Nexplanon.
Each implant is a plastic rod about the size of a matchstick. The rods contain a form of the hormone progesterone called etonogestrel.
A woman becomes pregnant when an egg released from her ovary (the organ that holds her eggs) is fertilized by a man's sperm. The fertilized egg attaches to the inside of a woman's womb (uterus), where it receives nourishment and develops into a baby. Hormones in the woman's body control the release of the egg from the ovary -- called ovulation -- and prepare the body to accept the fertilized egg.
Hormonal contraceptives (the pill, the patch, and the vaginal ring) all contain a small amount of man-made estrogen and progestin hormones. These hormones work to inhibit the body's natural cyclical hormones to prevent pregnancy. Pregnancy is prevented by a combination of factors. The hormonal contraceptive usually stops the body from ovulating. Hormonal contraceptives also change the cervical mucus to make it difficult for the sperm to go through the cervix and find an egg. Hormonal contraceptives can also prevent pregnancy by changing the lining of the womb so it's unlikely the fertilized egg will be implanted.
Another option for hormonal contraceptives is the extended-cycle pill, such as Seasonale, which was the first one to be approved. Seasonale contains the same hormones as other birth control pills, but the hormones are taken in a longer cycle. That reduces the number of menstrual periods from 13 periods a year to only four a year. That means a woman who takes this pill will menstruate only once each season.
Seasonale contains the same combination of two hormones that are commonly used in other hormonal contraceptives. But the pill is taken continuously for 12 weeks followed by one week of inactive pills, which results in a menstrual cycle. Other extended-cycle pills, such as Seasonique and LoSeasonique use a different configuration of hormones. Both of these pills use estrogen in the final week, with LoSeasonique providing a lower dose option.