Morning-after pills can help prevent pregnancy if you've had unprotected sex — whether you didn't use birth control, you missed a birth control pill or your method of birth control failed.
Morning-after pills can prevent pregnancy because conception typically doesn't occur immediately after sex. Instead, it may happen up to several days later. During the time between sex and conception, sperm travel through the fallopian tubes until they potentially reach an egg. As a result, using emergency contraception soon after unprotected sex isn't too late to prevent pregnancy.
Morning-after pills do not end a pregnancy that has implanted. Depending on where you are in your menstrual cycle, morning-after pills may act by one or more of the following actions: delaying or preventing ovulation, blocking fertilization, or keeping a fertilized egg from implanting in the uterus. However, recent evidence strongly suggests that Plan B One-Step and Next Choice do not inhibit implantation. It's not clear if the same is true for Ella.
Keep in mind that the morning-after pill isn't the same as mifepristone (Mifeprex), also known as RU-486 or the abortion pill. Mifeprex terminates an established pregnancy — one in which the fertilized egg has attached to the uterine wall and has already begun to develop.