I've never had one in, but have done some research, and it is still possible to get pregnant.  Only small chance, but your concerns should be discussed with your gp to rule out pregnancy.  Apparently, all IUD's put you at greater risk of an ectopic pregnancy (a pregnancy that developes in the fallopian tubes) so it is better to get it checked out.

I Would go back to you Gp and see what they say,..

I have had the mirena for  4 years and stopped having my period all together after 6 weeks of having it in..

xx cazza

If you are worried go and get it checked.  However I have found on it that I don't really get proper periods - just regular spotting.  When I was worried at one stage I asked the dr about chances of it failing and he said as long as it was properly in place the chance of a pregnancy was less then one percent, so a very very very small chance of it not doing its job.  Hope that makes you feel a bit better?

My impression after having one inserted is that over time, the bleeding may stop altogether. Which I am hoping will do for me. If you are at all concerned, go and see your doctor.