All IUDs are so effective against pregnancy, they oddly can prevent pregnancy even if actually close to the uterus! That was not how they were designed to work. And do not let this discussion deter you from this excellent form of contraception as they are excellent contraception, and misplacement is not common!
Paragard IUDs occupy the cavity of the uterus, and thus prevent pregnancy through physical prevention of sperm and egg meeting. IUD mechanism of action has been argued from the stand point of whether there is also prevention of implantation. Most studies indicate that is not the primary way pregnancy is prevented. Mirena IUDs prevent heavy bleeding menstrual periods by their hormone, and also prevent pregnancy this way. If the Mirena is dislodged, the hormone can still prevent pregnancy, even if residing in the abdomen and not in the uterus! January 14th issue of Contraception they have a case report of a woman who was unable to conceive until after the intrabdominal Mirena was removed.
