An intrauterine device, or IUD, is a small, T-shaped device inserted into the uterus to prevent pregnancy. It works by either releasing hormones or using copper to create an environment that is not conducive to sperm fertilizing an egg. IUDs can be effective for several years, depending on the type. There are two main types of IUDs: hormonal and copper. Hormonal IUDs release progestin, which thickens cervical mucus and thins the uterine lining, while copper IUDs use copper to immobilize sperm. Both types are considered highly effective forms of long-term contraception.