If you’ve already ovulated, Plan B is unlikely to prevent pregnancy. The drug works almost entirely by blocking or delaying ovulation, so once an egg has been released, it has little to no effect. This is one of the most important limitations of Plan B that many people don’t realize until they need it.
How Plan B Actually Works
Plan B contains a synthetic hormone called levonorgestrel that blocks the surge of luteinizing hormone (LH) your body uses to trigger ovulation. If you haven’t ovulated yet, suppressing that hormonal signal keeps the egg from being released, which means sperm have nothing to fertilize. That’s the entire mechanism. It stops the process before it starts.
Despite older label language and persistent misconceptions, Plan B does not prevent a fertilized egg from implanting in the uterus. A 2022 review in the journal Contraception examined the available research and found that nine out of ten studies showed no difference in uterine receptivity after levonorgestrel exposure compared to controls. The review concluded there is no evidence that Plan B taken after ovulation prevents implantation. It also does not thicken cervical mucus or create a barrier to sperm in any meaningful way.
This means Plan B has essentially one job: stop ovulation. If that window has passed, the drug doesn’t have a backup mechanism to fall back on.
What Happens If You’ve Already Ovulated
When levonorgestrel is given after the LH surge has already begun, pooled data from three randomized trials found it performed no differently than a placebo in preventing the egg’s release. Only 14.6% of cycles treated with levonorgestrel had ovulation delayed by five or more days, compared to 4% with placebo, a gap too small to be statistically meaningful at that late stage. If Plan B doesn’t inhibit ovulation, the resulting conception rates are similar to what you’d expect from completely unprotected sex during your most fertile window.
Sperm can survive in the reproductive tract for three to five days, according to the Mayo Clinic. So if you had unprotected sex a day or two before ovulation and then took Plan B after the egg was already released, live sperm could still be present and capable of fertilization. The pill wouldn’t change that outcome.
The Problem With Knowing When You Ovulate
Here’s the complication: most people don’t know exactly when they ovulate. Calendar-based tracking methods are only about 75% effective, and they require months of consistent monitoring to establish a pattern. Even then, ovulation can shift by several days due to stress, illness, travel, or irregular cycles. Cycles shorter than 26 days or longer than 32 days make calendar predictions especially unreliable.
Ovulation predictor kits that detect the LH surge are more precise, but they tell you ovulation is about to happen, not that it already has. By the time you’re looking back and trying to figure out whether you’d ovulated before taking Plan B, the answer is often genuinely uncertain. If you’re unsure whether you’ve ovulated, taking Plan B is still reasonable because there’s a chance it could still work. But if you have strong reason to believe ovulation already occurred (positive ovulation test a day or two prior, for example), you should know that Plan B’s effectiveness drops significantly.
Options That Work Later in the Cycle
If you’re concerned that ovulation has already happened or is imminent, two alternatives offer better protection.
Ella (ulipristal acetate) is a prescription emergency contraceptive pill that works differently from Plan B. It can still delay ovulation even after the LH surge has started. In the same pooled trial data, ulipristal acetate delayed ovulation for at least five days in 59% of cycles when given during the late follicular phase. Even when administered during the LH surge itself, it prevented egg release in 79% of cycles, compared to just 14% for levonorgestrel and 10% for placebo. The median time from treatment to ovulation was six days with ulipristal versus just two days with levonorgestrel or placebo. Ella requires a prescription, but some telehealth services can provide one quickly.
The copper IUD is the most effective form of emergency contraception available. It can be inserted up to five days after unprotected sex, and when the day of ovulation can be estimated, it can be placed even later, as long as insertion occurs within five days of ovulation. Unlike hormonal methods, the copper IUD works primarily by creating an environment in the uterus that is inhospitable to fertilization. It’s highly effective regardless of where you are in your cycle, and it doubles as long-term birth control for up to ten years.
Body Weight Affects Plan B’s Effectiveness
Even when taken before ovulation, Plan B becomes less reliable at higher body weights. Research has shown that the drug is more likely to fail in people with a BMI above 26. Levonorgestrel is particularly sensitive to weight and BMI compared to other emergency contraception options. If this applies to you, ulipristal acetate or the copper IUD are more dependable choices regardless of cycle timing.
What to Expect After Taking Plan B
Whether or not Plan B successfully prevented ovulation, it can change your next period. Your cycle may come earlier or later than usual, and the flow might be heavier, lighter, or spottier than normal. Spotting between taking the pill and your next period is common. These changes typically resolve within one cycle, though using emergency contraception repeatedly can make periods less predictable over time.
If your period is more than a week late after taking Plan B, a pregnancy test is a good idea. This is especially true if you suspect you had already ovulated when you took it, since the pill’s effectiveness in that scenario is minimal. A standard home pregnancy test will be accurate by the time your period is a week overdue.