I had it for over a year when I started having symptoms but the tests would be very faintly positive and it turned out I was pregnant but when I had the IUD removed I lost the baby. So it is very possible to get pregnant on it, it's supposed to be a 1/100 chance to get pregnant on it but I'm not even the only person I know to get pregnant on it and when I went to my OB she said I was the second one to come in recently that had gotten pregnant with it still in place.
It isn't common to become pregnant with an iud in. Because Mirena is hormonal birth control though it can cause a lit of symptoms of being pregnant without you actually being pregnant. Its the reason I ended up having mine removed.
I dont know exact statistics but I had mirena and am currently 17wks pregnant. So, it happens
My best friend has one & had been feeling pregnant last week. I urged her to go to the dr & when she did they did an ultrasound & discovered she'd just had a miscarriage. It is possible to become pregnant with a mirena & it is quite dangerous for mum & bub, if you think you might be you need to go to your local ed straight away.