I have a few uterine fibroids that I had before my mirena was put in, the largest one being 4 cm. I have to say, that surprisingly, the mirena did not hurt ( or help) the fibroids. It ruined everything else for me, but not my fibroids. I was told that it usually helps shrink them, and helps if they bleed, but again, none of that happened to me, they are still the same after having the mirena in for 10 months.