Plan B delays ovulation by about 2 days on average, though the actual delay depends heavily on how close you are to ovulating when you take it. When taken early enough in your cycle, it can push ovulation back by 5 or more days in some cases, but its ability to delay ovulation drops sharply the closer you are to releasing an egg.
How Plan B Delays Ovulation
Plan B contains a large dose of levonorgestrel, a synthetic hormone that works by interfering with the signal your brain sends to trigger egg release. Normally, a surge of luteinizing hormone (LH) tells your ovary it’s time to release a mature egg. Plan B suppresses that LH surge, which delays the entire ovulation process. Without the hormonal green light, the egg stays put.
This is essentially Plan B’s only trick. Research has consistently shown it has little to no effect after ovulation has already occurred. In one study of 17 women who took it after ovulation, the expected number of pregnancies matched the observed number almost exactly, meaning the pill provided no meaningful protection. Plan B works by preventing ovulation, not by interfering with anything that happens afterward.
Timing Changes Everything
The size of your developing egg follicle at the time you take Plan B largely determines whether the pill will work. FDA review data breaks this down clearly:
- Follicle 12 to 14 mm (several days before ovulation): Plan B prevented follicle rupture within 5 days in 83% of women, compared to 56% on placebo. At this stage, it suppressed the LH surge in 94% of women.
- Follicle 15 to 17 mm (approaching ovulation): Plan B still suppressed the LH surge in 91% of women, but this hormonal suppression didn’t reliably translate into preventing the egg from actually releasing.
- Follicle 18 mm or larger (ovulation imminent): Plan B suppressed the LH surge in only 47% of women. At this point, the ovulatory process is already underway and difficult to stop.
The key takeaway: Plan B is most effective when taken several days before you would have ovulated. Once your body has already begun the LH surge, Plan B rarely prevents the follicle from rupturing. In pooled clinical data, the median delay from treatment to follicle rupture was just 2 days with Plan B, compared to 6 days with ella (a different emergency contraceptive). Only about 15% of Plan B cycles showed a delay of 5 or more days.
Why the 5-Day Window Matters
Sperm can survive inside the reproductive tract for up to 5 days. The goal of emergency contraception is to delay ovulation long enough that any surviving sperm die before an egg becomes available. If Plan B pushes ovulation back by only a day or two, sperm from unprotected sex might still be viable when the egg finally releases. This is why the pill is more effective the sooner you take it after sex, and why it struggles when you’re already on the verge of ovulating.
How Ella Compares
Ella (ulipristal acetate) works through a different mechanism. Instead of simply suppressing the LH surge, it blocks progesterone receptors, which allows it to interfere with ovulation at a later stage. In pooled data from three clinical trials, ella delayed ovulation by at least 5 days in about 59% of cycles, compared to roughly 15% for Plan B.
The most striking difference appears when the LH surge has already started. At that critical moment, ella still prevented follicle rupture in 79% of cases. Plan B prevented it in just 14%, which was nearly identical to the 10% seen with placebo. Neither pill worked once LH levels had fully peaked, but ella buys significantly more time during the narrow window when it matters most.
Body Weight and Effectiveness
Plan B’s ability to delay ovulation appears to weaken at higher body weights. Multiple analyses suggest reduced effectiveness in people weighing 165 pounds (75 kg) or more, or with a BMI at or above 26. For those with a BMI of 30 or higher, the reduction in efficacy is more pronounced. Ella holds up better across a wider weight range, though it’s not completely unaffected. A copper IUD remains the most effective and weight-neutral emergency contraception option regardless of body size.
Effects on Your Next Period
Because Plan B works by disrupting your normal hormonal cycle, it commonly shifts the timing of your next period. Your period may come a few days early or a few days late. The further you were from ovulation when you took the pill, the more likely it is to arrive early, since the hormonal disruption can trigger shedding of the uterine lining sooner than expected. If you were closer to ovulation, a delayed period is more common, since the pill pushed back the entire second half of your cycle. Heavier or lighter bleeding than usual is also normal. If your period is more than a week late, a pregnancy test is a reasonable next step.