It is important to differentiate between the emergency contraceptive pill and the abortion pill.
While the former is taken to prevent a pregnancy from occurring, the latter is to be used to abort or terminate a pregnancy that has already occurred. While one is viewed principally as a form of contraception, the other is an abortifacient.
The view of the majority of the medical community is that since it is the function of the morning after pill to prevent a pregnancy from occurring, it will have no effect on an existing pregnancy; i.e. it will not terminate the pregnancy.
Though some scientists believe that EC may possibly act after fertilization, a possibility that leads some to consider EC an abortifacient, if the pregnancy occurs after intercourse but before the pill is taken, then the pill will not work.
Another question that occurs at this point is what is the effect of the Morning After pill if it is taken before intercourse? Actually the morning after pill is a bit of a limiting term and not entirely accurate, because the pill is as effective taken shortly before intercourse as after it.
When taken after intercourse the pill can be taken up to 72 hours after intercourse. The WHO says they can be used for up to 5 days after contraceptive failure.