Contraceptive coil. Nurse holding a box containing a hormonal intrauterine device (IUD, coil). An IUD is a small contraceptive device that is inserted into the uterus in order to prevent fertilisation. It can contain either copper or levonorgestrel (synthetic progestogen hormone, as here). The hormone thickens the cervical mucus, making it harder for sperm to reach the uterus and in some cases 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.