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, Next Choice One Dose, Next Choice, and Levonorgestrel Tablets) 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 2010 study 3 of ulipristal acetate (ella) found that at certain doses, it can decrease the thickness of the endometrium (by 0.6 to 2.2 mm), but it is not clear that this would in fact prevent the implantation of a fertilized egg.
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. Stratton P, Levens E, Hartog B, Piquion J, Wei Q, Merino M, Nieman L. Endometrial effects of a single early luteal dose of the selective progesterone receptor modulator CDB-2915. Fertility and Sterility 2010;93:2035-2041.
1 Statement on Contraceptive Methods. Washington DC: American College of Obstetricians and Gynecologists, July 1998.