Answers to Frequently Asked Questions About...
How Emergency Contraception Works
How does emergency contraception prevent pregnancy?
Emergency contraceptive pills—like all regular hormonal contraceptives such as the birth control pill, the implant Implanon, the vaginal ring NuvaRing, the Evra patch, and the injectable Depo-Provera1, and even breastfeeding—prevent pregnancy primarily, or perhaps exclusively, by delaying or inhibiting ovulation and inhibiting fertilization. We can’t always completely explain how contraceptives work, and it is possible that any of these methods may at times inhibit implantation of a fertilized egg in the endometrium. But the best evidence that we have suggests that levonorgestrel and ulipristal acetate EC does not interfere with post-fertilization events.
The Copper-T IUD does not affect ovulation, but it can prevent sperm from fertilizing an egg. It may also prevent implantation of a fertilized egg.
Emergency contraceptive pills will not cause an abortion. EC is 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.
1 Statement on Contraceptive Methods. Washington DC: American College of Obstetricians and Gynecologists, July 1998.