Plan B is significantly less effective on the day of ovulation. The drug works by delaying or blocking the release of an egg, so if ovulation has already started or is imminent, it has little to no ability to prevent pregnancy. In clinical trials where women took Plan B after ovulation had begun, 17.7% became pregnant, compared to 0% of women who took it before ovulation.
How Plan B Actually Works
Plan B contains a synthetic hormone called levonorgestrel that interferes with the hormonal chain reaction your body uses to release an egg. Specifically, it blunts the surge of luteinizing hormone (LH) that triggers ovulation. By suppressing that surge, the drug can delay or prevent the egg from leaving the ovary, which means sperm that are waiting in the reproductive tract never get the chance to fertilize anything.
This is the only meaningful way Plan B prevents pregnancy. The FDA has concluded that Plan B has no direct effect on fertilization or implantation. It does not terminate a pregnancy, and it does not prevent a fertilized egg from attaching to the uterus. Its effectiveness depends entirely on whether it can stop or postpone ovulation.
Why Ovulation Day Is the Problem
If you’re ovulating right now, the LH surge has already peaked. Plan B can’t undo a surge that’s already happened. Research pooling data from three clinical trials found that when levonorgestrel was given during the late fertile window (around the time of the LH surge), it delayed ovulation for five or more days in only 14.6% of cycles. That was statistically no different from placebo, which delayed ovulation in 4% of cycles. On the day of the LH peak itself, no emergency contraceptive pill was effective at postponing egg release.
The pregnancy numbers tell the same story. In two trials that carefully timed ovulation using follicle measurements and hormone data, none of the 137 women who took Plan B before ovulation became pregnant. Among the 62 women who took it after ovulation, 11 became pregnant. That 17.7% pregnancy rate is close to what you’d expect with no emergency contraception at all during the fertile window.
The Timing You Can’t See
The tricky part is that most people don’t know exactly when they’re ovulating. Ovulation predictor kits detect the LH surge, but the surge can begin 24 to 36 hours before the egg is actually released. Symptoms like mittelschmerz (a twinge of pain on one side) or changes in cervical mucus offer rough clues, not precision. So when someone says “I think I’m ovulating today,” they could be anywhere from a day before the LH peak to a day after egg release.
This uncertainty matters because Plan B’s effectiveness drops sharply across a very narrow window. If you’re a day or two before the LH peak, the drug still has a reasonable chance of working. If you’re at or past the peak, it likely won’t help. There’s no reliable way to distinguish between those scenarios at home.
Plan B’s Effectiveness by Timing
The general statistics you’ll see about Plan B reflect averages across all cycle days, not just ovulation day. Those numbers look like this:
- Within 24 hours: reduces pregnancy risk by about 95%
- 25 to 48 hours: reduces risk by about 85%
- 49 to 72 hours: reduces risk by about 58%
These percentages compare pregnancy rates to what would happen with no intervention at all (roughly 8 pregnancies per 100 women after unprotected sex during the fertile window). But these are population averages that include women at every point in their cycle. For someone who is definitively ovulating, the real-world effectiveness is far lower than these numbers suggest, because the drug’s mechanism simply doesn’t apply once the egg is on its way out.
Ella Works Closer to Ovulation
A prescription emergency contraceptive called ella (ulipristal acetate) is significantly more effective near ovulation than Plan B. In the same pooled analysis, ella delayed ovulation for at least five days in 58.8% of cycles, compared to Plan B’s 14.6%. Even when given during the LH surge (not the peak, but the rising phase), ella prevented egg release in 79% of cycles versus 14% for Plan B.
Ella works through a different mechanism. Rather than just blunting the LH surge, it can block the progesterone receptors that the ovary needs to complete the process of releasing the egg. This gives it a wider window of effectiveness. However, even ella failed to prevent ovulation when given on the day of the LH peak itself. No pill-based emergency contraception reliably works at that exact point.
Both Plan B and ella have similar effectiveness when taken within three days of unprotected sex, but ella has a clear advantage in the four-to-five-day window. If you believe you’re close to ovulation, ella is the stronger option, though you’ll need a prescription or a telehealth visit to get it.
The Copper IUD Option
The most effective emergency contraception regardless of cycle timing is the copper IUD. In a study of 1,840 women who had it inserted after unprotected sex, including 850 who were in their expected fertile window, there were zero pregnancies at the one-month follow-up. The copper IUD works differently from pills: copper ions create an environment that is toxic to sperm and also affect the uterine lining, giving it effectiveness that doesn’t depend on blocking ovulation.
A copper IUD can be placed up to five days after unprotected sex, or even later if a provider can estimate that placement falls within five days of ovulation. CDC guidelines from 2024 confirm this extended placement window. The device then doubles as long-term contraception for up to 10 years if you choose to keep it. The downside is that it requires a clinic visit for insertion, which may not be feasible on short notice for everyone.
Body Weight and Effectiveness
Plan B may also be less effective for people who weigh more than 165 pounds or have a BMI over 25. Health Canada has gone so far as to state the drug won’t work above that weight threshold, though some reproductive health experts argue that’s an overstatement, since there isn’t strong enough data to say efficacy drops to zero. Still, if you’re near or above that weight range and also near ovulation, both factors work against you, making ella or the copper IUD even more worth considering.
What This Means Practically
If you’ve had unprotected sex and believe you’re ovulating today, Plan B is unlikely to significantly reduce your pregnancy risk. The drug’s entire mechanism depends on stopping an egg from being released, and on ovulation day, that process is already underway or complete. Sperm can survive in the reproductive tract for up to five days, so even if Plan B could slightly delay ovulation, viable sperm may still be present when the egg eventually releases.
Your most effective options near ovulation are ella (if you can get a prescription quickly) or a copper IUD (if you can get an appointment within the next few days). If Plan B is the only option available to you, taking it is still reasonable since you can’t be 100% certain of your exact ovulation timing, and if you’re even slightly earlier in the process than you think, it could still help. But the evidence is clear that its reliability drops dramatically at this point in the cycle.