How does an IUD work?
Both the ParaGard® and the Mirena® IUDs affect the way sperm move, preventing them from joining with an egg. If sperm cannot join with an egg, pregnancy cannot happen. Both types also alter the lining of the uterus. Some people say that this keeps a fertilized egg from attaching to the lining of the uterus. But there is no proof that this actually happens. The progestin in the Mirena® IUD helps prevent pregnancy. Progestin works by keeping a woman’s ovaries from releasing eggs — ovulation. Pregnancy cannot happen if there is no egg to join with sperm. Progestin also prevents pregnancy by thickening a woman’s cervical mucus. The mucus blocks sperm and keeps it from joining with an egg.
