How Plan B Prevents Pregnancy
Plan B (levonorgestrel) works primarily by delaying or preventing ovulation—the release of an egg from the ovary—when taken before the preovulatory LH surge occurs. 1, 2
Primary Mechanism of Action
Levonorgestrel stops or delays ovulation by blocking the midcycle LH surge from the pituitary gland. 2 This is the dominant mechanism through which pregnancy prevention occurs:
- When administered at least 2 days before ovulation, levonorgestrel effectively delays or prevents the egg from being released 2
- If given when ovulation is imminent or has already occurred, the medication cannot prevent ovulation and effectiveness drops significantly 2
- The drug works before the egg is released, not after fertilization or implantation 1
What Plan B Does NOT Do
There is no evidence that levonorgestrel prevents implantation or affects an existing pregnancy. 2 This is a critical distinction:
- Plan B will not work if you are already pregnant 1
- It will not affect an existing pregnancy 1
- Research shows no data supporting that levonorgestrel can impair embryo development or prevent implantation 2
Secondary Possible Mechanisms
Levonorgestrel may also affect sperm migration in the female reproductive tract and potentially impact fertilization, though these are less well-established mechanisms. 3
Time-Dependent Effectiveness
The sooner Plan B is taken after unprotected intercourse, the more effective it becomes, because timing relative to ovulation is critical. 1, 2
- Maximum effectiveness occurs within 72 hours (3 days) of unprotected sex 1, 4
- Efficacy decreases significantly after 72 hours, with pregnancy rates increasing at 4-5 days post-intercourse 4, 5
- The clinical observation that effectiveness decreases with longer intervals between intercourse and administration supports that the primary mechanism is preventing fertilization by blocking ovulation 2
Overall Efficacy
When taken as directed within 72 hours, Plan B prevents approximately 7 out of 8 expected pregnancies (about 80% efficacy). 1, 3 This moderate efficacy reflects the narrow window during which the medication can successfully delay ovulation before it naturally occurs.