Like the regular birth control pill, the main goal is to prevent ovulation. However, like the regular pill, and like some IUD's, one of the ways that it works is by making the endometrium in the uterus inhospitable to life. Therefore, if a woman has already ovulated, it is possible for the egg to be fertilized, but for it to not be able to implant in the uterus. If the fertilized egg does manage to implant itself, or if an egg has already implanted itself, it will not cause a miscarriage.