The IUD is a small, T-shaped, plastic device that is inserted into and left inside the uterus. There are two types of IUDs:
1. The hormonal IUD releases progestin. One hormonal IUD is approved for use for up to 5 years. Another is approved for use for up to 3 years.
2. The copper IUD does not contain hormones. It is approved for use for up to 10 years.

How does the IUD work?
Both types of IUDs work mainly by preventing fertilization of the egg by the sperm. The hormonal IUD also thickens cervical mucus, which makes it harder for sperm to enter the uterus and fertilize the egg, and keeps the lining of the uterus thin, which makes it less likely that a fertilized egg will attach to it.

Why gynecologists love IUDs
IUDs are an excellent birth control option because they are effective, safe and easy to use, said Dr. Sara Pentlicky, a gynecologist and family planning specialist at the University of Pennsylvania.
While some women can't use estrogen-containing birth control because of health issues, "there are very few women who can't use an IUD," Pentlicky said. She estimated that 80 percent of the female doctors in her practice use IUDs for their own contraception.
IUDs have to be inserted by a doctor, but once in place, they are effective immediately and can protect against pregnancy for five to 12 years, depending on the type.
Unlike birth control pills, which require that users remember to take them on a daily basis, IUDs need little to no maintenance. They are nearly 99 percent effective, according to a study published in May in the New England Journal of Medicine.
IUDs also differ from birth control pills in that women have a greater chance of becoming pregnant immediately after stopping use.
In the U.S., there are two IUDs available — ParaGard, a copper, hormone-free device that can protect against pregnancy for up to 12 years, and Mirena, which releases small amounts of a synthetic progestin hormone and can be effective for up to 5 years.
"With ParaGard, you don't actually stop ovulating like you do with the pill, so when I take it out, you should be able to get pregnant the next month without any trouble," Pentlicky said.



References:
The IUD: What Do Gynecologists Know That Other Women Don't? | Birth Control Use
Long-Acting Reversible Contraception (LARC): IUD and Implant - ACOG