An intrauterine device or IUD is a form of reversible contraception that consists of a T-shaped insert made up of either copper or a progesterone delivery system which is inserted into the uterus.
Copper IUDs work by impairing the motility of sperm. Progesterone IUDs work by preventing menstruation.
In Damned If You Do, Sister Mary Augustine suffered an allergic reaction to her copper IUD, which she had been using for contraception many years before.
Intrauterine device at Wikipedia