Answers to Frequently Asked Questions About...
How Emergency Contraception Works
How does emergency contraception prevent pregnancy?
Researchers have identified several ways that hormonal emergency
contraceptive pills (also called "morning
after pills" or "day after pills") likely prevent
pregnancy. How they might work in your case depends on where you
are in your monthly menstrual cycle when you use them. But no matter
when you take emergency
contraception, it will not
cause an abortion. For more about how EC works, read this article in Journal of the American Medical Association.
Studies show that both progestin-only and combined emergency contraceptive pills can prevent or delay ovulation (the
time in your cycle when your ovaries release an egg). If you take
emergency contraceptive
pills before fertilization (the point when the egg and sperm meet),
they may interfere with the process of fertilizing the egg, for instance
making it harder for the egg or the sperm to travel (and meet up)
in your reproductive tract. It’s also possible that emergency
contraceptive pills work after fertilization, making it impossible
for the fertilized egg to implant in your uterus; however, the best available evidence suggests that ECPs’ ability to prevent pregnancy can be fully accounted for by mechanisms that do not involve interference with post-fertilization events.
The Copper-T IUD does not affect ovulation, but like emergency contraceptive pills, it can prevent sperm from fertilizing an egg. It may also prevent implantation of a fertilized egg.
Hormonal emergency contraceptive pills are not the same as the abortion
pill. There is no time when the emergency contraceptive pills available in the United States would end
a pregnancy once it has started. Hormonal emergency contraceptive pills
don’t have any effect if you are already pregnant. If you decide
to have use an IUD for emergency contraception, your health care provider
would test you first to confirm you are not already pregnant.
Click here 
for information about research showing how emergency contraception
works and more details about the possible mechanisms of action.