If you take Plan B while you’re already ovulating, it is unlikely to prevent pregnancy. Plan B works by delaying or blocking ovulation, so once that process is already underway, the pill has little to no effect. A pooled analysis of three randomized trials found that when given near the time of ovulation, levonorgestrel (the active ingredient in Plan B) was no different from a placebo in preventing the egg’s release. Only 14% of cycles showed a delayed ovulation with Plan B at that stage, compared to 10% with a sugar pill.
This doesn’t automatically mean you’ll get pregnant, but it does mean Plan B isn’t meaningfully improving your odds of avoiding pregnancy at that point in your cycle.
Why Plan B Stops Working at Ovulation
Plan B is a synthetic progestin that prevents pregnancy by suppressing the hormonal surge that triggers egg release. When you take it early enough in your cycle, before that surge begins, it can effectively delay ovulation by days, pushing it past the window when sperm could still be viable. But this mechanism has a hard cutoff. Once your body’s hormonal cascade is in motion and the egg is preparing to release, Plan B can no longer interrupt the process.
Updated FDA labeling reflects this clearly: there is no evidence that levonorgestrel has any effect after ovulation. It does not prevent a fertilized egg from implanting, and it does not disrupt an existing pregnancy. Its entire mechanism is limited to delaying or preventing ovulation. So timing is everything.
How to Tell if You’re Ovulating
The tricky part is that most people don’t know exactly when they’re ovulating unless they’re actively tracking. Ovulation typically happens about 14 days before your next period starts, but this varies widely depending on cycle length and regularity. Signs that ovulation may be happening or imminent include a slight rise in basal body temperature, changes in cervical mucus (it becomes clear and stretchy, similar to egg whites), and mild pelvic cramping on one side.
If you use ovulation predictor kits, a positive result means the hormonal surge has begun, which is precisely the point at which Plan B loses its advantage. If you’ve already gotten a positive result and then had unprotected sex, Plan B is unlikely to help.
Ella Works Better Near Ovulation
A prescription emergency contraceptive called ella (ulipristal acetate) is significantly more effective than Plan B when taken close to ovulation. In the same pooled trial data, ella delayed ovulation for at least five days in 59% of cycles when given in the late fertile window. Even when administered at the time of the hormonal surge itself, ella prevented egg release in 79% of cycles, compared to just 14% for Plan B.
The reason for this difference is that ella works through a different mechanism. Rather than simply adding synthetic hormone to suppress the surge, it blocks the receptor that progesterone uses to trigger final egg release. This gives it a wider effective window. However, no oral emergency contraceptive is 100% effective once the surge has peaked. On the day of the hormonal peak itself, neither ella nor Plan B reliably stopped ovulation in trials.
Ella requires a prescription, but many telehealth services can provide one quickly. CDC guidelines note that while Plan B and ella perform similarly within the first three days after unprotected sex, ella is more effective in the 3-to-5-day window.
The Copper IUD Is the Most Reliable Option
If you’re concerned that you were ovulating when you had unprotected sex, a copper IUD is the most effective form of emergency contraception available. It prevents over 99% of pregnancies when inserted within five days of unprotected intercourse. In a study of 134 women who had a copper IUD placed 6 to 14 days after unprotected sex, zero pregnancies occurred.
The copper IUD works differently from pills. Rather than preventing ovulation, it creates an environment in the uterus that is inhospitable to sperm and fertilization. This means it remains effective even after ovulation has already happened, which is why it’s the go-to recommendation when timing is uncertain. It can also be left in place as long-term contraception for up to 10 years.
Getting one placed requires a clinic visit, but many reproductive health clinics offer same-day or next-day appointments for emergency contraception. CDC guidelines specifically note that when the day of ovulation can be estimated, a copper IUD can be placed even beyond the standard five-day window after sex, as long as it’s inserted within five days of ovulation itself.
Body Weight Can Further Reduce Plan B’s Effectiveness
Research suggests that Plan B may be less effective for people at higher body weights, which compounds the timing problem. Studies have found reduced effectiveness in individuals weighing 165 pounds (75 kg) or more, or with a BMI of 26 to 30 or above, though the exact thresholds vary across studies. People with higher body weight show lower blood concentrations of levonorgestrel after taking it, along with weaker suppression of ovulation near the hormonal surge.
Doubling the dose does not solve this problem. Randomized trial data show that taking two Plan B pills does not improve ovulation suppression in individuals with obesity. For people in higher weight ranges who are also near ovulation, ella or a copper IUD are more reliable choices.
What to Expect Afterward
Whether or not Plan B was effective, you may notice changes to your next period. It could arrive earlier or later than expected, and the flow might be heavier, lighter, or spottier than usual. These shifts are among the most common side effects and don’t tell you anything definitive about whether pregnancy was prevented.
If your period hasn’t arrived within three weeks of taking the pill, take a pregnancy test. A negative result at that point is generally reliable. If you took Plan B knowing you might be ovulating, monitoring for your period and testing if it’s late is especially important, since the pill’s effectiveness was likely reduced.