Scientists have identified several ways that emergency contraceptive pills likely prevent pregnancy. How they work in your case will depend on where you are in your monthly cycle and the emergency contraceptive you choose. They may prevent or delay ovulation just as regular birth control pills would if they are taken early in a woman's cycle. If ovulation has already occurred, they may interfere with fertilization of an egg or the implantation of a fertilized egg in the uterus. ECPs won't work if implantation has already taken place, which is why it's important to take them within 72 hours of unprotected sex, if possible. That is also the reason ECPs are not considered an abortion; they have no effect on a pregnancy already established.