Plan B delays ovulation by roughly a few days, though the exact number varies depending on where you are in your cycle when you take it. The pill works by flooding your body with a synthetic hormone that temporarily puts the brakes on the process leading to egg release. If you take it early enough in your cycle, before your body has committed to releasing an egg, it can push ovulation back long enough for sperm (which survive up to five days) to die off before they ever meet an egg.
How Plan B Actually Prevents Pregnancy
Plan B contains a large dose of levonorgestrel, a synthetic version of progesterone. Under normal circumstances, a surge of luteinizing hormone (LH) triggers your ovary to release a mature egg. Levonorgestrel interferes with that hormonal surge, either suppressing it entirely or blunting it enough that the follicle holding the egg doesn’t rupture on schedule. The delay doesn’t need to be long. Even pushing ovulation back by two or three days can be enough, because sperm have a limited survival window inside the reproductive tract.
A comprehensive review published in the journal Contraception confirmed that levonorgestrel works almost entirely by postponing or inhibiting ovulation. Nine out of ten studies found no meaningful change to the uterine lining’s ability to accept a fertilized egg, and the review found essentially no evidence of any effect after ovulation has already happened. In practical terms, Plan B is a pre-ovulation tool.
Why Timing in Your Cycle Matters So Much
The earlier in your fertile window you take Plan B, the more effectively it can delay ovulation. If you take it days before your LH surge begins, the pill has the best chance of fully suppressing that surge and keeping the egg locked in the follicle. If you take it when the LH surge is already underway or your follicle is close to its mature size, the pill’s ability to intervene drops sharply. Women who took levonorgestrel at or after the LH surge showed similar ovulation rates and conception rates to women who took nothing at all.
This is the core reason Plan B becomes less effective the longer you wait. It’s not just about the clock ticking after sex. It’s about how close your body is to releasing an egg. Two people could have unprotected sex on the same calendar day, take Plan B at the same time afterward, and get very different results based purely on where each person is in her cycle.
What Happens If You’ve Already Ovulated
If the egg has already been released, Plan B is unlikely to prevent pregnancy. A pilot study tracking women who took levonorgestrel after confirmed ovulation found that among those who had sex during their fertile window and took the pill one to two days after ovulation, the number of pregnancies observed matched almost exactly the number expected without any intervention. Three to four pregnancies were predicted; three occurred. Animal studies in rats and monkeys have reinforced this, showing no effect of high-dose levonorgestrel on events after fertilization.
This is why the common advice to take Plan B “as soon as possible” isn’t just about general urgency. Every hour you wait is an hour closer your body may be to ovulating, and once that happens, the pill loses its primary mechanism of action.
Weight Can Reduce Effectiveness
Your body weight significantly affects how well Plan B works. Research shows that the standard 1.5 mg dose hits a ceiling of effectiveness at around 70 kg (about 154 pounds), and for individuals weighing 80 kg (176 pounds) or more, levonorgestrel-based emergency contraception may have essentially no efficacy. People with a BMI of 30 or higher who used Plan B had more than four times the risk of pregnancy compared to those with a BMI under 25.
The reason is pharmacological: at higher body weights, the drug gets diluted. People with a BMI of 30 or above achieved peak blood concentrations of levonorgestrel roughly 50% lower than those with a BMI under 25. Some research has explored whether doubling the dose to 3 mg corrects this gap, and early pharmacokinetic data suggests it does, but a double dose isn’t currently the standard recommendation. If you weigh over 155 pounds, a copper IUD or ella (ulipristal acetate) are generally more reliable emergency contraception options.
How Plan B Affects Your Next Period
Because Plan B works by disrupting your normal hormonal rhythm, it commonly shifts the timing of your next period. The pattern is fairly predictable: the earlier in your cycle you take it, the earlier your period arrives. Women who took Plan B during the first three weeks of their cycle experienced significantly shorter cycles, with menstrual bleeding starting sooner than their usual schedule. Those who took it later, during week three or after, didn’t see their period shift much in timing but did tend to have more bleeding days in the following cycle.
Other side effects are usually mild and short-lived, typically lasting only a few days. The most common ones include nausea, fatigue, headache, breast tenderness, dizziness, and cramping. Some people experience light spotting between periods. Vomiting can occur, which is worth noting because if you throw up within two hours of taking the pill, it may not have been fully absorbed.
The Practical Takeaway on Timing
Plan B pushes ovulation back by a matter of days, not weeks. That short delay is enough to work because it outlasts the survival time of sperm waiting in the reproductive tract. But the pill can only create that delay if your body hasn’t already crossed the hormonal point of no return. Taking it within the first 24 hours after unprotected sex gives you the best odds, not because the drug works differently at that point, but because you’re more likely to still be in the window where ovulation can be interrupted. By 72 hours, effectiveness drops considerably, and beyond that, the standard levonorgestrel pill isn’t recommended at all.