Plan B is significantly less effective if you’ve already ovulated. The pill works primarily by delaying or preventing ovulation, so if an egg has already been released, Plan B has largely missed its window. Research finds no evidence that levonorgestrel (the active ingredient in Plan B) prevents a fertilized egg from implanting in the uterus, which means that once ovulation has occurred, conception rates are similar to what you’d expect from unprotected sex during your most fertile days.
How Plan B Actually Works
Plan B contains a high dose of levonorgestrel, a synthetic hormone that stops or delays the release of an egg from the ovary. It does this by suppressing the hormonal surge that triggers ovulation. If that surge hasn’t started yet, Plan B can delay ovulation by over two weeks, effectively moving the egg release far enough from the time sperm entered your body that fertilization can’t happen. Sperm survive about 3 to 5 days inside the reproductive tract, so this delay strategy works well when timed correctly.
The problem is timing. In a study that gave women levonorgestrel right around ovulation, only 4 out of 12 had their ovulation significantly delayed, and just 1 didn’t ovulate at all. The remaining 8 women ovulated normally, as if they hadn’t taken the pill at all. Researchers found that in those 8 women, the only measurable effect was a shorter luteal phase (the second half of the cycle), which doesn’t prevent pregnancy.
Why Timing Matters So Much
Your fertility window spans roughly six days: the five days before ovulation (because sperm can wait) plus the day of ovulation itself. Plan B is most effective when taken before ovulation, ideally before the hormonal surge that triggers egg release has begun. Once that surge is underway, the pill’s ability to intervene drops sharply. And once the egg is actually out, the primary mechanism of the drug no longer applies.
This is why Plan B’s overall effectiveness numbers can be misleading. The commonly cited figure of around 87% effectiveness is an average across all cycle days. For women who happen to take it well before ovulation, the effectiveness is much higher. For women who take it on the day of ovulation or after, the research suggests it offers little to no additional protection beyond what chance alone would provide.
Does Plan B Prevent Implantation?
You may have heard that Plan B has a “backup” mechanism: preventing a fertilized egg from attaching to the uterine lining. This claim appears on some older product labels, but the scientific evidence doesn’t support it. A comprehensive review in the journal Contraception examined the available research and concluded that there is no evidence levonorgestrel taken after ovulation prevents implantation. The original research articles supporting a post-ovulatory effect simply don’t exist in the literature.
This matters practically, not just ethically. It means that if you’ve already ovulated and an egg has been fertilized, Plan B is unlikely to change the outcome. It also means that for people who have concerns about medications that might affect a fertilized egg, Plan B does not appear to work that way.
More Effective Alternatives
If you’re concerned that you may have already ovulated, two alternatives offer better protection.
The first is ella (ulipristal acetate), a prescription emergency contraceptive pill. It’s more effective than Plan B at delaying ovulation even when the hormonal surge has already started. Overall efficacy estimates for ella range from 62% to 85%, compared to 47% to 53% for levonorgestrel. It can be taken up to 5 days after unprotected sex.
The second, and most effective option regardless of cycle timing, is a copper IUD. Unlike pills, it doesn’t rely on delaying ovulation. The copper creates an environment that is toxic to both sperm and eggs, and it can be inserted up to 5 days after unprotected sex. In some guidelines, it can be placed even later if the timing of ovulation can be reasonably estimated. Pregnancy rates with the copper IUD for emergency contraception are consistently lower than with any oral method. It also doubles as long-term birth control for up to 10 years if you choose to keep it.
Body Weight Can Reduce Effectiveness
Plan B’s effectiveness also drops for people above certain weight thresholds. Clinical guidelines from the UK’s Faculty of Sexual and Reproductive Healthcare note that levonorgestrel emergency contraception appears less effective in women with a BMI above 26 or a weight above 70 kg (about 154 pounds). If this applies to you and you’re also near ovulation, the compounding factors make Plan B a particularly unreliable choice. Ella or a copper IUD would be stronger options in this situation.
What to Expect Afterward
Regardless of whether Plan B “worked,” you’ll likely notice some changes in your cycle. Your next period may come earlier or later than expected, and it may be heavier, lighter, or spottier than usual. A delayed period is one of the most common side effects, which can be anxiety-inducing when you’re already worried about pregnancy.
The key benchmark: if your period doesn’t arrive within 3 weeks of taking Plan B, take a pregnancy test. Home tests are accurate by that point. Don’t rely on symptoms alone, since early pregnancy symptoms and Plan B side effects (nausea, breast tenderness, fatigue) overlap considerably.
If you took Plan B while ovulating and are now within the first few days afterward, it’s still worth considering whether a copper IUD is an option for you. It remains effective for emergency contraception up to 5 days after unprotected intercourse and, depending on the clinical situation, potentially longer.