If you take Plan B during or after ovulation, it is unlikely to prevent pregnancy. Plan B works primarily by delaying or blocking ovulation, so once an egg has already been released, the drug has little to no effect. Clinical data show that when Plan B is taken after ovulation, pregnancy rates are essentially the same as if no emergency contraception were taken at all.
How Plan B Actually Works
Plan B contains a synthetic hormone that prevents pregnancy by suppressing or delaying ovulation. When taken during the first half of your cycle, before the hormonal surge that triggers egg release, it can effectively stop or postpone that release long enough for sperm to die off (sperm survive about five days in the reproductive tract). The FDA’s review of clinical data confirms that Plan B is effective only when taken during this pre-ovulation window, when it can still interfere with follicle development and suppress the hormonal surge.
This is a narrower mechanism than many people realize. Plan B does not block fertilization after an egg is released, and current evidence shows it does not prevent a fertilized egg from implanting. A 2022 review in the journal Contraception found that women who took Plan B at or after the hormonal surge had conception rates identical to women who took a placebo. In practical terms, if the egg is already on its way, Plan B doesn’t change the outcome.
The Problem With Timing
The challenge is that most people don’t know exactly when they ovulate. Ovulation typically happens around day 14 of a 28-day cycle, but it varies widely. Stress, sleep, illness, and irregular cycles can shift it by days. Even ovulation predictor kits detect the hormonal surge that precedes egg release by only 24 to 36 hours, meaning by the time you see a positive result, your window for Plan B’s effectiveness may already be closing.
If you had unprotected sex and suspect you’re near or past ovulation, the key question is whether the hormonal surge has already started. If it has, Plan B performs no better than a placebo at preventing the egg from being released. Pooled data from three randomized trials found that Plan B delayed egg release in only about 15% of cycles when given during this late-stage window, compared to 4% with a placebo. That difference was not statistically significant.
Ella Is More Effective Near Ovulation
A prescription alternative called Ella works through a different mechanism and remains substantially more effective closer to ovulation. In the same pooled analysis, Ella delayed egg release in about 59% of late-stage cycles, compared to Plan B’s 15%. Even when taken during the hormonal surge itself, Ella still prevented ovulation in 79% of cases, while Plan B managed only 14%.
There is one hard limit: neither drug was effective when taken on the actual peak day of the hormonal surge. But because Ella’s effective window extends further into the fertile period, it’s the better option when you suspect ovulation is imminent or already underway. Ella requires a prescription in the U.S., though some telehealth services can provide one quickly.
A copper IUD is the most effective emergency contraception regardless of where you are in your cycle. Inserted within five days of unprotected sex, it reduces pregnancy risk by more than 99% and works even after ovulation has occurred.
Body Weight Can Reduce Effectiveness Further
Plan B’s effectiveness drops at lower body weight thresholds than many people expect. Clinical data show a sharp increase in pregnancy rates for women weighing over 70 kg (about 154 lbs) or with a BMI above 26. For context, a BMI of 26 is just barely into the “overweight” category. If you’re above either threshold and also near ovulation, Plan B’s already limited protection decreases even more. Clinical guidelines in this situation suggest Ella as a first choice, or a copper IUD.
What to Expect Afterward
Even if Plan B doesn’t prevent pregnancy, taking it won’t harm a pregnancy that occurs. It also won’t cause an abortion. The drug simply loses its ability to do anything useful once ovulation has happened.
Plan B can shift the timing of your next period. It may arrive up to a week later than expected, or it could come a few days early. Spotting between your period and the expected date is also common. These changes happen because the synthetic hormone temporarily disrupts your cycle’s normal hormonal pattern, regardless of whether the pill successfully prevented ovulation.
If your period is more than a week late, take a pregnancy test. If you’re unsure when your period is due, test at least 21 days after the unprotected sex. Home pregnancy tests are highly accurate by that point. Testing earlier than this can produce a false negative because pregnancy hormone levels may not yet be high enough to detect.