Answers to Frequently Asked Questions About...
How Emergency Contraception Works
How does emergency contraception prevent pregnancy?
Emergency contraceptive pills prevent pregnancy primarily, or perhaps exclusively, by delaying or inhibiting ovulation. There is no evidence to suggest that either of the FDA-approved emergency contraceptive options, levonorgestrel (LNG, such as Plan B One-Step, Take Action, Next Choice One Dose or My Way ) or ulipristal acetate (UPA, such as ella) works after an egg is fertilized. In two recent studies1,2 of the levonorgestrel regimen, women who presented for EC were monitored to identify their menstrual cycle day and ascertain which women took EC before ovulation, and which took it after. Among women taking LNG before ovulation, there were no pregnancies. Among those who took it on the day of ovulation or after, there were about the number of pregnancies that would be expected with no use of EC; if LNG were effective at preventing implantation, it would most certainly be more effective when taken after ovulation. A recent study of UPA EC showed that when UPA was used before ovulation, the number of pregnancies that occured was significantly lower than would be expected without EC, while when it was used after ovulation, the pregnancy rate was what would be expected without EC.3
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 point in a woman's cycle 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 first confirm that 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. Novikova N, Weisberg E, Stanczyk FZ, Croxatto HB, Fraser, IS. Effectiveness of levonorgestrel emergency contraception given before or after ovulation - a pilot study.Contraception 2007;75: 112-118.
2. Noé G, Croxatto HB, Salvatierra AM, Reyes V, Villarroel C, Muñoz C, Morales G, Retamales A. Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation. Contraception 2011;84:486-492.
3. Li HWR, Lo SST, Ng EHY, Ho PC. Efficacy of ulipristal acetate for emergency contraception and its effect on the subsequent bleeding pattern when administered before or after ovulation. Hum Reprod. 2016;31:1200-07.
1 Statement on Contraceptive Methods. Washington DC: American College of Obstetricians and Gynecologists, July 1998.