There is always a small chance you can become pregnant
with an
IUD. A large number of the reported pregnancies
with the IUD result in ectopic pregnancies. But a very small number of pregnancies have resulted in utero with the IUD.
They can move, migrate into the uterus or surrounding tissue, although again it is rare. They can also on occation fall out. Be sure to check your strings periodically, and monitor for pain.