Plan B, also known as the morning-after pill, is an emergency contraceptive pill taken by mouth after unprotected sex to prevent pregnancy. However, it will not end an established pregnancy. If a woman takes Plan B mistakenly during pregnancy, it will not harm the developing fetus. Emergency contraception works by temporarily stopping ovulation, so theres no egg for sperm to meet up with and fertilize. Therefore, it is not effective after pregnancy has occurred and cannot interrupt it. If a woman is already pregnant and does not want to be, she may be able to get an abortion. It is important to note that the morning-after pill is very different from the abortion pill.