Mazal tov on the recent birth of your baby!
If possible, we strongly recommend waiting until after you immerse to have the Mirena inserted. Irregular staining is common, as is bleeding due to the insertion, and both could cause difficulty completing the clean days.
If you have the Mirena inserted after you immerse, any bleeding that takes place within the first few days after the insertion may be attributed to trauma and would not render you niddah. Additionally, you may take precautions against becoming niddah from further staining by wearing colored underwear or pantyliners and waiting a few seconds (ideally 15 seconds) after urinating before wiping.
In order to delay insertion of the Mirena until after mikveh, you can discuss with your physician using a barrier method of contraception for a short period of time. If you are exclusively breastfeeding, your chances of pregnancy at this point are quite low and the combination of this decreased fertility and spermicide is reasonable protection. Alternatively, you can immerse to allow physical contact but avoid having marital relations until the Mirena is inserted.
If you do have the Mirena inserted before you immerse, there are a number of suggestions we can offer to try to help you successfully complete the clean days. You should change your white underwear more frequently to prevent stains from accumulating to the size of a gris. This way stains smaller than a gris on your underwear may be disregarded and do not invalidate your clean days. Additionally, you may reduce the number of bedikot required to the hefsek taharah and one bedikah per day throughout the clean days. If even that is difficult due to consistent staining, you may reduce further to the hefsek taharah, one bedikah each on days 1, 7, and one bedikah on an intermediate clean day. You should also take care to wait at least a few seconds (ideally 15 seconds) after urinating before wiping. This way you may disregard stains found on the toilet paper.
While there may be little you can do to prevent the staining associated with the Mirena, you can try to manage the situation using the suggestions and leniencies described above. Nevertheless, you should be prepared for the possibility of a difficult beginning as your body adjusts to the Mirena.
Please feel free to get back to us with any further questions.
B’Hatzlacha!
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.