An intrauterine device (IUD) is a method of birth control that is placed inside the uterus to prevent pregnancy. It is not clearly understood how the IUD prevents pregnancy, and different types of IUDs may prevent pregnancy in different ways.

All IUDs have a plastic string that extends through the opening of the cervix into the vagina. The string allows a woman to check that the IUD is in place. It also allows a health professional to remove the device at the woman's request or at the end of its effectiveness (usually anywhere from 5 to 10 years, depending on the type).

The IUD is very effective at preventing pregnancy.