The intrauterine device (IUD) is a small T-shaped flexible device inserted into the uterine cavity.  IUDs can be inert, copper-releasing or progestin-releasing. Copper-releasing IUDs interfere with the ability of sperm to pass through the uterine cavity and with the reproductive process before ova reach the uterine cavity. Progestin-releasing IUDs also thicken the cervical mucus and change the endometrial lining.