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. IUDs prevent sperm from fertilizing an egg, and prevent fertilized eggs from implanting in the uterus. The use of IUDs is also being studied in the prevention and treatment of endometrial cancer and other conditions. Also called intrauterine device.