OKAY, here is the DEAL!
BOTH IUD'S do not prevent ovulation, you do still ovulate.
COPPER is a potent spermicide, it makes their heads explode. The copper IONS inside of the uterus kill sperm. With this being being said, it also *can* prevent implantation.
I become pregnant on an IUD, and later had an abortion. Obviously it does not 'abort' all pregnancies, otherwise I would not have had a problem.
The MIRENA IUD thins the uterine lining and increases cervical mucousa. Thick cervical mucousa prevents sperm from being able to move upward through the cervix! NO sperm in uterus, no pregnancy...
The Mirena IUD only prevents ovulation in about 45% of women using it.
The Mirena IUD *can* also prevent implantation.
IUD'S MAIN action is always supposed to be either KILLING sperm, or ensuring that sperm cannot even reach the uterus in the first place.