Good point Keike.

It's not impossible to get pregnant on Mirena - one of my uni friends did - but sadly, they are usually not viable pregnancies and will either miscarry naturally or need a medical termination becuase they won;t usually settle in the right spot bcuase the Mirena is in the way.
This happened to my Mum, with one of the horrible old IUD's in the 70's. For this reason and also the other side effects, about the only option I'll have is a diaphragm.