Whether IUD insertion itself renders a woman niddah is subject to halachic debate. Our site's position is that the procedure is generally carried out with a very small instrument and does not render a woman niddah.
The question that remains is whether any bleeding that results over the next few days can be ascribed to trauma from the insertion, in which case it would not render a woman niddah& or invalidate her clean days.
Our site's rabbinic supervisor, Rav Yehuda Henkin, rules that bleeding not immediately following insertion does render a woman niddah, unless she can establish medically that the blood is from uterine trauma. To establish this medically, either the doctor would have to say he caused uterine trauma consistent with the bleeding, or such trauma would have to be visible on inspection. Ideally, at insertion, a woman should ask her physician if he caused trauma or sees bleeding from the insertion.
After IUD insertion, whether there was trauma or not, the leniencies of stains do apply. So we recommend reviewing the laws of stains (as by reading our site's section on stains) beforehand.
Please write back with any further questions.