Intrauterine device. Illustration of an intrauterine contraceptive device (IUD) in place in the uterus. The IUD typically consists of a small piece of plastic that is inserted into the uterus (triangular space) through the cervix (below). A piece of plastic string hangs through the cervix into the vagina (wide, at bottom left), allowing easy removal. Exactly how IUDs work is not understood, but they are thought to prevent implantation of fertilized eggs in the wall of the uterus. The probability of a pregnancy for each year of use is 2 to 3 percent. At right are a paired fallopian tube and ovary.