High Quality Content by WIKIPEDIA articles! The modern intrauterine device (IUD) is a form of birth control in which a small `T`-shaped device, containing either copper or progesterone, is inserted into the uterus. IUDs are a form of long-acting reversible contraception, which is the most effective type of reversible birth control. As of 2002, IUDs were the most widely used form of reversible contraception, with nearly 160 million users worldwide.