Menstrual cycle length can change with age and due to the effect of hormones. It is common to experience irregular bleeding during the first 3-6 months on the Mirena – although not necessarily the short, regular cycles you describe. You did not mention how long you have been on the Mirena, but if you are still in the first few months after insertion, you may want just to allow your body some more time to adjust and seeing if your cycles become longer.
If you had the Mirena inserted more than 6 months ago and your cycles have only started to get shorter during the past 3 months, we recommend speaking to your doctor about whether hormonal intervention can delay your periods. If your doctor does not think this will help lengthen your cycles, you might want to consider having the Mirena removed and trying a different method of birth control. See our article on choosing a contraceptive method for more details.
Please note that many women have very light periods on the Mirena that may not render them niddah. If your period is not actually a flow, rather it is just spotting (i.e., you don’t need a pad or tampon for the bleeding, just a light pantyliner), you may take precautions against becoming niddah from the spotting. See our articles on stains and toilet paper for more details.
Please feel free to get back to us with any further questions.
B’Hatzlacha!