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In case, you are within the fertile period of your cycle, then the chances of pregnancy, after the mirena IUD is out - are higher. Hence you should use a back-up form of birth control like condoms or emergency contraceptive pills to prevent pregnancy.
In case, this is not followed, and you are concerned about a possible pregnancy, you can always take a pregnancy test, which will clear all your doubts.
You should consult with your doctor, and once pregnancy is ruled out, Mirena may be replaced within 7 days of a menstrual period.
I hope this helps.