MODEL RELEASED. Mirena IUD contraceptive being held by a woman. This intrauterine device (IUD) is a T-shaped piece of plastic that releases low doses of progesterone. It is inserted in the uterus (womb) to prevent pregnancy. The presence of the device alone is effective at preventing a fertilised egg implanting in the uterine wall. The addition of the female sex hormone progesterone increases the reliability of the device. The hormone thickens the cervical mucus, making it harder for sperm to reach the uterus. In some cases it also stops ovulation. It also causes lighter and shorter periods, and so may be used to treat heavy periods (menorrhagia). Progesterone is released for up to 5 years, after which the device is removed, using the thread at top.