IUD - The IUD is a specially shaped piece plastic that must be insert into the uterus by a doctor. Although it is not known for sure how it works, it is believed that an IUD changes the lining of the uterus to prevent the fertilized egg from being implanted. It may also slow the sperm down enough to prevent it from reaching the uterus. Some IUDs are covered with very fine copper wire. Others contain a hormone that is slowly released. A doctor will select the best one suited for the patient.