Yes it would affect your periods. Mirena is an intra uterine device that is used for the prevention of pregnancy. It contains levnorgestrol that prevents the fertilization of the egg or sperm and reduces the lining the endometrium. It is a highly effective contraceptive that can be used up to five years. While using a Mirena birth control it can bring about a few changes in the body. The periods may appear for longer days, have heavier or light bleeding, spotting. This usually lasts till the body gets adjusted to this hormone-releasing contraceptive and can range from 3-6 months following, which the period will return to normal. I would suggest you to not to get worried and in case of increased heavier bleeding which doesn't subside then you can consult your obg-gyn doctor.