How Does Plan B One-Step Work?
Depending upon where you are in your cycle, Plan B One-Step may work in one of these ways:
It may prevent or delay ovulation.
It may interfere with fertilization of an egg.
It is also possible that this type of emergency birth control prevents implantation of a fertilized egg in the uterus by altering its lining.
Plan B One-Step is not the same as RU-486, which is an abortion pill. It does not cause a miscarriage or abortion. In other words, it does not stop development of a fetus once the fertilized egg implants in the uterus. So it will not work if you are already pregnant when you take it.www.webmd.com