Can I Take Norethisterone While On Mirena?

Hi i have been prescibed Northisterone 5mg 3 times a day because i had a mirena coil fitted and have had it removed because we want to try for a baby. the reason i was put on Northisterone is none stop bleeding for last 3 months since having the coil removed, they are brilliant on stopping the bleeding and have to take them for 21 days then a 7 day break can i still conceive because we really want a baby and im not understanding how i can become pregnant