Could you tell me what happens if you get pregnant while using mirena (levonorgestrel) (iud birth control)?

Possible Miscarriage. It increases the chances of a miscarriage, so the sooner it can be easily removed the better. If the string is not reachable and it can not be easily removed, do nothing. Although you may miscarry, you also may complete a normal healthy pregnancy.
Removal. Your gyecologist will need to do a pelvic exam and an ultrasound. She will remove the iud and possibly send you to a high risk gynecology specialist.