A small, plastic T-shaped device that is placed inside the uterus (the small, hollow, pear-shaped organ in a woman’s pelvis in which a fetus develops) to prevent pregnancy. Intrauterine devices prevent sperm from fertilizing an egg, and prevent fertilized eggs from implanting in the uterus. The use of intrauterine devices is also being studied in the prevention and treatment of endometrial cancer and other conditions. Also called IUD.