Intrauterine device. Coloured X-ray of an intrauterine device (IUD, pink) in place in the uterus or womb of a 29 year old woman. This is a form of contraception that is thought to work by preventing the implantation of a fertilised egg in the uterine lining. It comprises a T-shaped piece of metal or plastic. The IUD is not quite as effective as the contraceptive pill, but does not have the mental and physical side-effects associated with taking hormone drugs. The sacrum bone is seen above the IUD at centre, with the two blades of the pelvis at upper left and right.