Levonorgestrel is a female hormone that can cause changes in your cervical mucus and uterine lining, making it harder for sperm to reach the uterus and harder for a fertilized egg to attach to the uterus. Levonorgestrel intrauterine system is a plastic device that contains this hormone. This device is placed in the uterus where it slowly releases the hormone to prevent pregnancy for 3 to 5 years. Mirena is meant for use in a woman who has had at least one child. Skyla can be used whether you have children or not. Mirena is also used in women who have heavy menstrual bleeding and choose to use an intrauterine form of birth control. Levonorgestrel is a progestin hormone and does not contain estrogen. The intrauterine device release levonorgestrel in the uterus, but only small amounts of the hormone reach the bloodstream. Levonorgestrel intrauterine system may also be used for purposes not listed in this medication guide.