An IUD does not fall out during your period because of the way it is placed in your uterus. Unlike your uterine lining, which is tissue that sloughs off during your period, an IUD is a firm object and has two arms that make it fit snugly in your uterus. It it also difficult for it to go through your cervical os (a term for the tip of your cervix, where the IUD strings go through) since the cervical os is pretty closed and too tight for the IUD to go through on its own, whereas blood can go through this os during your period.
