What can you tell me about the mechanism of action of emergency contraception?

For standard levonorgestrel EC pills (LNG) the only mechanism for which there is evidence of action is inhibition and delay of ovulation, and prevention of fertilization.

Timing is everything, as described in a recent review by Gemzell-Daniellsson (see abstract below.) A peak in the pituitary hormone LH initiates ovulation. LNG has an impact only if taken about 2 or 3 days before that LH peak, and not afterward. (Thus the importance of taking EC as early as possible.) Ulipristal acetate which is the compound used in the new EC ella, appears to have a somewhat longer window of effectiveness.

In terms of other mechanisms of action, per the article, LNG EC does not have an impact on the endometrium during the expected time of implantation. And in an in vitro test by the authors, LNG had no effect on blastocyst (the fertilized egg after several cell divisions) viability, or attachment and early implantation. Moreover, the lack of effectiveness of LNG-EC if taken after ovulation, during the several days when implantation would be taking effect, also indicates lack of impact on implantation.


A major barrier to the widespread acceptability and use of emergency contraception (EC) are concerns regarding the mechanisms of action of EC methods. Today, levonorgestrel (LNG) in a single dose of 1.5 mg taken within 120 h of an unprotected intercourse is the most widely used EC method worldwide. It has been demonstrated that LNG-EC acts through an effect on follicular development to delay or inhibit ovulation but has no effect once luteinizing hormone has started to increase. Thereafter, LNG-EC cannot prevent ovulation and it does not prevent fertilization or affect the human fallopian tube. LNG-EC has no effect on endometrial development or function. In an in vitro model, it was demonstrated that LNG did not interfere with blastocyst function or implantation

Gemzell-Danielsson K. Mechanism of action of emergency contraception. Contraception 2010; 82:404-9.

Dr. Jim Shelton

Science Advisor, Bureau for Global Health


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