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Mirena is a Hormonal preparation and can casue temporary hormonal imbalance in the body.Since the cycles have been regular it is expected that you must be ovulating.it is necessary that you have regular intercourse during ovulation days which are 10 to 16 days in a regular 28 to 30 day cycle and first day of period is counted as day one.
Continue having regular intercourse.
Consider visiting a Gynaecologist for hormonal analysis.
Take care!