Plan B works primarily by delaying ovulation, but the exact number of days it pushes ovulation back varies depending on where you are in your cycle when you take it. There’s no single universal number. What matters most is whether your body has already started the hormonal cascade that triggers egg release, because that determines whether the delay works at all.
How Plan B Delays Ovulation
Plan B contains a synthetic hormone called levonorgestrel that disrupts the process your body uses to release an egg. Normally, a surge of luteinizing hormone (LH) signals your ovary to release a mature egg. Levonorgestrel interferes with this signal, either suppressing the surge entirely or blunting it enough that normal ovulation doesn’t occur. The result is that your egg stays put, and without an egg available, sperm can’t cause a pregnancy.
In some cases, even when the egg does eventually release after a delay, the ovulatory process is disrupted enough that fertilization becomes much less likely. Researchers have described this as “ovulatory dysfunction,” where the egg is released under abnormal hormonal conditions that reduce the chances of conception.
The Delay Depends on Your Cycle Timing
If you take Plan B several days before ovulation, when the LH surge hasn’t started yet, it can delay egg release by roughly five to seven days. This is the window where Plan B is most effective, because it has time to suppress the hormonal chain reaction before it gains momentum. By the time your body resets and ovulation finally happens, any sperm from unprotected sex are no longer viable (sperm survive about five days in the reproductive tract).
If you take it closer to ovulation but still before the LH surge begins, the delay may be shorter, perhaps two to four days. The closer you are to that surge, the harder it is for levonorgestrel to fully suppress it.
Here’s the critical point: once the LH surge has already started, Plan B loses its ability to delay ovulation. Studies show that women who took levonorgestrel at or after the onset of the LH surge had similar rates of ovulation and similar conception rates as women who took no emergency contraception at all. This is why timing matters so much, and why Plan B is more effective the sooner you take it after unprotected sex.
Why There’s No Exact Number
Most people searching this question want a clean answer: “Plan B delays ovulation by X days.” The reason that number doesn’t exist is that ovulation timing varies from person to person and cycle to cycle. You likely don’t know exactly when you were going to ovulate, and you can’t easily tell whether your LH surge has already kicked in. Plan B can shift ovulation anywhere from a couple of days to about a week, but the shift is measured against a moving target.
What you can observe is its effect on your period. Research published in the journal Contraception found that taking Plan B in the pre-ovulatory phase (before egg release) shortened the overall cycle by about one day on average, while taking it in the post-ovulatory phase lengthened the cycle by close to two days. About one in four women experienced their cycle lengthening by two or more days, and about one in five had it shorten by two or more days. Your next period may arrive up to a week late, which is normal. If it’s more than five days late, a pregnancy test is a reasonable next step.
The Sooner You Take It, the Better It Works
Plan B is approved for use up to 72 hours after unprotected sex, but its effectiveness drops with every passing hour. This isn’t just about the drug wearing off. It’s about your body getting closer to ovulation with each passing day. The sooner you take it, the more likely it is that the LH surge hasn’t started yet, giving levonorgestrel the best chance of blocking or delaying egg release.
Weight Can Affect How Well It Works
Body weight significantly influences how effectively Plan B delays ovulation. Research shows that levonorgestrel-based emergency contraception begins losing effectiveness at around 70 kg (about 154 pounds) and may have essentially no efficacy at 80 kg (about 176 pounds) or above. Women with a BMI of 30 or higher had more than four times the risk of pregnancy compared to women with a BMI under 25.
This happens because the drug’s concentration in the bloodstream is lower at higher body weights, which means it may not reach the threshold needed to suppress the LH surge. If your weight falls in this range, a copper IUD placed within five days of unprotected sex is the most effective emergency contraception option regardless of body weight. Another oral option, ulipristal acetate (sold as ella), maintains better efficacy at higher weights than levonorgestrel, though it requires a prescription.
Taking Plan B More Than Once
If you’ve taken Plan B and then had another instance of unprotected sex later in the same cycle, you can take it again. Repeated use doesn’t reduce its effectiveness or cause long-term side effects. However, using it frequently can make your periods irregular and harder to predict, which makes it difficult to track your cycle or know when you’re fertile. It’s safe as a backup, but it’s not designed as a primary method of contraception.
Keep in mind that if Plan B delayed your ovulation by a few days after the first dose, you may actually be in your fertile window when you wouldn’t normally expect to be. This shifted timeline is easy to misjudge, which is one reason why reliable ongoing contraception is more practical than repeated emergency use.