Plan B doesn’t push ovulation back by a fixed number of days. Instead, it works by suppressing the hormonal surge that triggers egg release, effectively stalling ovulation for long enough that sperm in the reproductive tract die off before they can fertilize an egg. Sperm typically survive up to five days, so even a delay of a few days can prevent pregnancy. But whether Plan B delays ovulation at all depends entirely on where you are in your cycle when you take it.
How Plan B Actually Delays Ovulation
Ovulation is triggered by a sharp spike in luteinizing hormone (LH), sometimes called the LH surge, which signals a mature egg follicle in the ovary to rupture and release an egg. Plan B’s active ingredient, levonorgestrel, works by binding to progesterone receptors in the brain and disrupting the release of the hormones that cause this surge. Without that spike, the follicle stays intact and the egg isn’t released.
This isn’t a precise “pause” with a defined number of days. What happens is the normal ovulation timeline gets interrupted, and your body essentially has to restart the signaling process. Your next ovulation attempt may come several days later, or your cycle may simply be pushed back, resulting in a later period. Many women notice their period arrives a few days early or late after taking Plan B, which reflects this hormonal disruption.
Timing Matters More Than Anything Else
The most important factor isn’t how far back Plan B pushes ovulation. It’s whether ovulation has already been set in motion when you take it. Levonorgestrel only works if taken before the LH surge has begun. Once that surge is underway, Plan B cannot stop it.
This creates a narrow but critical distinction. In the days leading up to ovulation, when estrogen is rising but the LH surge hasn’t fired yet, Plan B is at its most effective. But in the 24 to 48 hours immediately before ovulation, when the LH surge is already happening, levonorgestrel performs no better than a placebo at preventing the egg from being released. Research pooling data from multiple studies found that when a dominant follicle had already reached 18 millimeters or larger (its pre-rupture size), levonorgestrel failed to prevent ovulation at rates statistically identical to a sugar pill.
This is why Plan B’s effectiveness drops sharply the longer you wait. It’s not just about the clock ticking. It’s that each passing hour brings you closer to the point in your cycle where the drug simply cannot do its job.
What Happens If You’ve Already Ovulated
If you take Plan B after ovulation has occurred, the evidence is clear: it does not prevent pregnancy through any other mechanism. A 2022 review examining multiple studies found that women who took levonorgestrel at or after the LH surge had conception rates similar to women who took a placebo. Nine out of ten studies in that review found no difference in how receptive the uterine lining was to a fertilized egg after Plan B exposure compared to controls.
In plain terms, Plan B does not interfere with implantation. Its only meaningful mechanism is delaying or blocking ovulation. If the egg has already been released, the drug has missed its window.
Body Weight Can Reduce Effectiveness
Plan B’s ability to delay ovulation also appears to weaken at higher body weights. The American College of Obstetricians and Gynecologists notes that levonorgestrel may be less effective for women with a BMI of 25 or higher, and effectiveness drops further at a BMI of 30 or above. The exact threshold isn’t perfectly defined, but the trend is consistent enough that clinical guidelines recommend women in higher weight ranges consider an alternative.
The most commonly recommended alternative is ulipristal acetate (sold as ella), which works through a similar but more potent mechanism. Unlike levonorgestrel, ulipristal can still suppress the LH surge even after it has started, as long as it hasn’t yet peaked. Studies show ulipristal can delay follicular rupture for at least five days, giving it a wider effective window. It also appears to retain effectiveness at higher body weights better than levonorgestrel does. Ulipristal requires a prescription, while Plan B is available over the counter.
Why There’s No Single Number
You’ll sometimes see claims that Plan B “delays ovulation by five days” or similar figures. That number actually comes from research on ulipristal acetate, not levonorgestrel. For Plan B specifically, studies have not identified a consistent delay measured in days. The effect depends on how close you are to ovulating, your individual hormone levels, and your body weight. For some women, it may push ovulation back by only a day or two. For others, taken early enough in the fertile window, it may delay it long enough to shift the entire cycle.
What the research does confirm is the principle behind the delay: if sperm are waiting in the reproductive tract (which they can do for up to five days), and Plan B can stall egg release long enough for those sperm to lose viability, pregnancy is prevented. The drug doesn’t need to delay ovulation by a specific number of days. It just needs to outlast the sperm.