Why it's done

Morning-after pills can help prevent pregnancy if you've had unprotected sex — either because you didn't use birth control, you missed a birth control pill, you were sexually assaulted or your method of birth control failed.

Morning-after pills do not end a pregnancy that has implanted. They work primarily by delaying or preventing ovulation.

Keep in mind that the morning-after pill isn't the same as mifepristone (Mifeprex), also known as RU-486 or the abortion pill. This drug terminates an established pregnancy — one in which the fertilized egg has attached to the uterine wall and has begun to develop.