Plan B typically delays ovulation by about 4 to 5 days when taken early enough in your cycle. That delay works by postponing the hormonal surge that triggers your ovary to release an egg, essentially pressing pause on the process long enough for sperm (which survive up to 5 days in the reproductive tract) to die off before an egg appears. The catch: timing matters enormously, and in some cases Plan B may not delay ovulation at all.
How Plan B Delays Ovulation
Your body releases an egg through a chain of hormonal events, the most important being a spike in luteinizing hormone (LH) known as the LH surge. Plan B’s active ingredient interrupts this chain before it reaches the tipping point, preventing or postponing the moment your follicle ruptures and releases an egg.
In one clinical study, women who took Plan B three days before their expected ovulation saw their LH and FSH peaks delayed by 4 to 5 days, and their overall cycle length stretched by roughly the same amount. Women who took it just one day before expected ovulation got far less benefit: the hormonal peak shifted by only about one day. In a separate study, when women took Plan B while their lead follicle was still small (12 to 14 millimeters), 83% showed no follicle rupture within five days, compared to 56% in a placebo group.
The practical takeaway: the earlier in your fertile window you take Plan B, the longer and more reliably it can delay ovulation.
When Plan B Can’t Delay Ovulation
Once the LH surge has already started, Plan B loses its ability to block or delay egg release. This is the single biggest limitation of the drug, and it’s not widely understood. A review published in Contraception found that women who took Plan B at or after the start of the LH surge had ovulation-inhibition rates and conception rates essentially identical to women who took a placebo. One study broke this down further: at the very start of the LH surge, Plan B prevented ovulation in 25% of women versus 0% on placebo. But before the LH peak (14% vs. 10%) and at the peak itself (9% vs. 4%), the differences were statistically insignificant.
Similarly, no study found a meaningful effect on ovulation when the lead follicle had already grown to 18 millimeters or larger. At that point, the body is essentially committed to releasing the egg, and Plan B can’t override the process. Because you can’t easily know where you are in your LH surge without specialized testing, this creates real uncertainty about whether Plan B will work for any given person on any given day.
Why the 4 to 5 Day Window Matters
Sperm can survive in the reproductive tract for up to five days. That’s why sex a few days before ovulation can still lead to pregnancy. Plan B’s strategy is straightforward: delay the egg’s release long enough that any surviving sperm have already died. A 4 to 5 day delay, when it works, covers almost the entire lifespan of sperm. A one-day delay, by contrast, may not be enough if intercourse happened several days before you took the pill.
This also explains why Plan B is most effective when taken as soon as possible. Every hour you wait, your body moves closer to the LH surge and the window where Plan B can meaningfully intervene gets smaller.
How Your Weight Affects the Delay
Body weight changes how much of Plan B’s active ingredient actually reaches effective levels in your bloodstream. Women with a BMI of 30 or higher achieve peak blood concentrations roughly 50% lower than women with a BMI under 25. That pharmacological difference translates directly into higher failure rates: women at a BMI of 30 or above had over four times the risk of pregnancy compared to women under a BMI of 25.
The data suggests Plan B’s effectiveness drops noticeably at around 70 kilograms (about 154 pounds) and may provide essentially no benefit at 80 kilograms (about 176 pounds) or above. UK clinical guidelines recommend doubling the dose to 3 milligrams for women at higher weights, which has been shown to correct the gap in blood concentration. If you weigh above these thresholds, a different type of emergency contraception may be a better option.
How Plan B Compares to Ella
Ella (ulipristal acetate) works through a similar ovulation-delaying mechanism but has two significant advantages. First, it maintains consistent effectiveness for up to 120 hours (five days) after unprotected sex, while Plan B’s effectiveness drops sharply after 48 hours. In the 72 to 120 hour window, Ella prevented all pregnancies in one trial of over 200 women, while three pregnancies occurred among those who took Plan B.
Second, Ella can delay ovulation even when the LH surge has already begun, a point in the cycle where Plan B is essentially no longer effective. This makes Ella a stronger option when you’re closer to ovulation or when several days have passed since unprotected sex. Ella requires a prescription in the United States, while Plan B is available over the counter.
What to Expect With Your Next Period
Because Plan B works by shifting your ovulation timing, it often shifts your period timing too. Your next period may come up to a week earlier or later than expected. Some people see no change at all. A period that arrives within a week of its usual date in either direction is considered normal after taking Plan B. If your period is more than a week late, a pregnancy test is a reasonable next step.