Mechanism of Action: Levonorgestrel Does NOT Prevent Implantation After Ovulation
Levonorgestrel emergency contraception works primarily by preventing or delaying ovulation—it does NOT prevent implantation of a fertilized egg if ovulation has already occurred. 1
Primary Mechanism: Ovulation Prevention
The FDA-approved labeling explicitly states that levonorgestrel "works mainly by preventing ovulation (egg release)" and may also prevent fertilization, but critically notes it may prevent "attachment of a fertilized egg to the uterus (implantation)" only as a theoretical possibility—not a proven mechanism. 1
Key mechanistic points:
Before ovulation occurs: Levonorgestrel blocks or delays the release of an egg from the ovary when given before the preovulatory LH peak. 1, 2
After ovulation has occurred: There is no evidence for an anti-implantation effect, despite this being "often postulated" in older literature. 2
Possible effects on fertilization: The medication may affect sperm migration in the female reproductive tract and potentially interfere with fertilization itself, but these are secondary mechanisms. 2
Clinical Implications for Timing
The effectiveness of levonorgestrel depends entirely on whether it is taken BEFORE ovulation:
Maximum efficacy occurs when taken as soon as possible within 72 hours of unprotected intercourse, with effectiveness declining significantly after 72 hours. 3, 4, 1
Pregnancy rates increase at 4-5 days post-intercourse because by this time, ovulation has likely already occurred in many women, and levonorgestrel cannot prevent pregnancy once the egg is released. 3, 4
The medication prevents approximately 7 out of 8 expected pregnancies when taken promptly, but this reflects its ability to prevent ovulation in women who would have ovulated after intercourse. 1
Why This Matters Clinically
If ovulation has already happened when levonorgestrel is taken, the medication will likely fail to prevent pregnancy:
The 20% failure rate (preventing ~80% of pregnancies) largely reflects cases where ovulation occurred before the medication was administered. 2
For women presenting 3-5 days after intercourse, ulipristal acetate (UPA) is significantly more effective than levonorgestrel because it can delay ovulation even closer to the LH surge. 3, 4, 5
The copper IUD remains effective up to 5 days post-intercourse (or 5 days after ovulation) because it has a true post-fertilization mechanism that levonorgestrel lacks. 3, 6
Common Misconception to Avoid
Critical caveat: Many patients and providers mistakenly believe emergency contraception works by preventing implantation of an already-fertilized egg. This is scientifically unfounded for levonorgestrel. 2 The medication's effectiveness window corresponds to the time before ovulation typically occurs, not to any post-fertilization effect.
Bottom line for counseling: If a patient asks whether levonorgestrel will work after ovulation has occurred, the honest answer is that it likely will not, because its mechanism requires preventing ovulation in the first place. 1, 2