How does the Mirena IUS work?

The Mirena IUS / IUD is inserted into a woman’s uterus by a doctor to provide long-term birth control. Once in place, it slowly releases a synthetic version of the hormone progestin called “levonorgestrel.” This synthetic hormone thickens the cervical mucus, which blocks sperm from reaching an egg. The hormone also thins the lining of the uterus, which reduces the ability of a fertilized egg to implant itself and grow.