I thought it kept a fertilized egg from implanting?
@BooBear:
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How Does the Morning-After Pill (Emergency Contraception) Work?
Pregnancy doesn't happen right after sex. That's why it's possible to prevent pregnancy even after the fact. It can take up to six days for the sperm and egg to meet after having sex. Emergency contraception pills work by keeping a woman's ovary from releasing an egg for longer than usual. Pregnancy cannot happen if there is no egg to join with sperm.
You might have also heard that the morning-after pill causes an abortion. But that's not true. The morning-after pill is not the abortion pill. Emergency contraception is birth control, not abortion.
-From planned parenthood website 4951