Plan B Effectiveness After Menses
Yes, Plan B (levonorgestrel) is effective if taken after finishing menses, but its effectiveness depends critically on when intercourse occurs relative to ovulation and how quickly it is taken after unprotected sex.
Key Mechanism and Timing
Plan B works primarily by preventing or delaying ovulation 1. The critical insight from research is that levonorgestrel emergency contraception is highly effective when taken before ovulation occurs but has little to no effect once ovulation has already happened 2, 3, 4.
When It Works Best
- ECPs should be taken as soon as possible within 5 days of unprotected sexual intercourse 1
- Effectiveness is highest within the first 3 days, with pregnancy rates increasing at 4-5 days 1
- The expected pregnancy rate of 8% (without contraception) is reduced to approximately 1% with Plan B 5
Timing Relative to Your Menstrual Cycle
The effectiveness after finishing menses depends on when ovulation occurs in your cycle:
Early in Cycle (Days 1-7 after menses starts)
- High effectiveness - You are typically far from ovulation
- Plan B can effectively prevent or delay the upcoming ovulation
- Research shows that when taken before ovulation (Days -5 to -2 of the fertile window), expected pregnancies were prevented entirely 2
Mid-Cycle (Around Days 8-19 in a typical 28-day cycle)
- Variable effectiveness - Depends on proximity to ovulation
- The fertile window varies greatly even among women with regular cycles 1
- If taken after ovulation has already occurred, effectiveness drops dramatically 2, 3
Critical Research Finding
Studies demonstrate that all pregnancies that occurred despite Plan B use happened when women took it after ovulation had already occurred 2, 3. Specifically, women who had intercourse on Days -1 to 0 (around ovulation) and took Plan B on Day +2 became pregnant, while those who took it before ovulation did not 2.
Important Caveats
Reduced effectiveness in certain populations:
- May be less effective in obese women (BMI ≥30) 1
- Consider ulipristal acetate (ella) as an alternative, which maintains effectiveness 3-5 days after intercourse and may be more effective in heavier women 1, 6
- Use barrier contraception for 7 days after taking Plan B
- Take a pregnancy test if your period is delayed more than one week beyond expected date
- Seek medical attention for severe lower abdominal pain 3-5 weeks after taking Plan B (to rule out ectopic pregnancy)
Menstrual changes are common 5:
- 87% of women have their next period within ±7 days of expected time
- 13% experience delays >7 days
- Bleeding patterns may vary (74% normal, 14% heavier, 12% lighter)
Bottom Line
Plan B remains effective after finishing menses, particularly if unprotected intercourse occurs in the early part of your cycle before ovulation. However, the sooner you take it after unprotected sex, the better it works 5. If you're concerned about timing relative to ovulation or have a higher BMI, discuss ulipristal acetate or copper IUD insertion with your healthcare provider, as these may offer superior effectiveness 1.