MEDICAL ABORTION Plan B One-Step (up to 72 hours) Plan B One-Step is also called the morning after pill. The pill contains 1.5 milligrams of levonorgestrel. Depending on where you are in your menstrual cycle, the pill may prevent ovulation, interfere with fertilization, or prevent implantation of a fertilized egg in the uterus. Plan B One-step is not effective if implantation has already occurred. Mifeprex (mifepristone) (up to 49 days after 1st day of last menstrual cycle) Formerly known as RU-486, Mifeprex is used with misoprostol to end a pregnancy. It does so by blocking the action of the hormone progesterone. Progesterone is needed to sustain a pregnancy.