How Emergency Contraception Works

Emergency contraception prevents pregnancy by stopping or delaying release of an egg (ovulation), blocking fertilization by affecting the egg or sperm, or preventing implantation by making the lining of the uterus inhospitable for pregnancy. Pregnancy is defined by the medical community as implantation of a fertilized egg in the lining of a woman’s uterus, so emergency contraception works before pregnancy has occurred. A pregnancy test is not useful because emergency contraception is used to prevent pregnancy rather than to abort an existing pregnancy. A woman who uses emergency contraception will never know if a pregnancy would have otherwise occurred. Fertility returns with the next cycle unless a birth control method is continued.

In order for emergency contraceptive pills to be effective, they must be taken as directed.