IUDs. A variety of different intrauterine contraceptive devices. IUDs consist of a piece of moulded plastic that is inserted into the uterus. Some types have a coil of copper wire wrapped around the stem, and most have a piece of plastic string that hangs through the cervix, 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 2-3%. IUDs with copper coils need to be replaced every 3-5 years.