If you take Plan B while you’re actively ovulating, it is unlikely to prevent pregnancy. Plan B works primarily by delaying or blocking ovulation, so once your egg has already been released, the drug has largely missed its window. The best available evidence suggests that Plan B works only before ovulation, and its ability to interfere with the process drops sharply as ovulation approaches.
Why Plan B Loses Effectiveness Near Ovulation
Plan B contains a synthetic hormone called levonorgestrel. Its main job is to suppress or delay the hormonal surge that triggers your ovary to release an egg. When taken early enough in your cycle, before your body has committed to releasing that egg, it can effectively hit pause on the process. But this ability deteriorates rapidly as ovulation gets closer.
The research on this timing is strikingly clear. In one study, when women took levonorgestrel two days before ovulation, 93% still ovulated normally, showing both follicle rupture on ultrasound and the rise in progesterone that confirms an egg was released. Another study found that 84% of women ovulated normally when they took the drug after their follicle had already reached 18 mm or larger (the size it reaches just before release). A third study found that women who took it around two days before ovulation experienced no delay in ovulation whatsoever.
In short, by the time you’re within a day or two of ovulation, Plan B can no longer reliably stop the egg from being released. If you’re already ovulating when you take it, that window has closed entirely.
Does Plan B Affect Fertilization or Implantation?
Some people wonder whether Plan B might still work after ovulation by blocking a fertilized egg from implanting in the uterus. A systematic review of the evidence found that nine out of ten studies showed no difference in the uterine lining’s receptivity to implantation after levonorgestrel exposure compared to controls. The review concluded that taking levonorgestrel after ovulation does not affect implantation and results in conception rates similar to placebo.
This means Plan B taken during or after ovulation offers essentially no additional protection compared to taking nothing at all.
How High Is the Pregnancy Risk?
The day of ovulation itself carries roughly a 33% chance of conception from a single act of unprotected sex. That probability is the highest of any point in the cycle. For comparison, intercourse five days before ovulation carries about a 10% chance. If Plan B cannot meaningfully reduce your odds once ovulation has occurred, you’re facing that baseline risk without effective backup.
Even under ideal circumstances (taken well before ovulation and soon after sex), Plan B is 81 to 90% effective. That rate drops the longer you wait after intercourse and the closer you are to ovulation. These two factors compound: if you had sex right around ovulation and waited 48 hours to take Plan B, the protection is minimal at best.
How to Tell If You’re Ovulating
Most people won’t know exactly when they’re ovulating unless they’ve been tracking specific signs. In the days just before ovulation, cervical mucus becomes clear and slippery, similar to raw egg whites. After ovulation, your basal body temperature rises by about 0.5 to 1 degree Fahrenheit, though this is only detectable if you’ve been measuring your temperature first thing each morning before getting out of bed.
Ovulation predictor kits detect the hormonal surge that precedes egg release, typically giving you a 12- to 36-hour heads up. If you got a positive result on one of these kits around the time you had unprotected sex, that’s a strong signal that Plan B may not be effective. Keep in mind, though, that most people reaching for emergency contraception haven’t been actively tracking, so you may not know for certain where you are in your cycle.
More Effective Options If You’re Near Ovulation
If you suspect you’re close to or past ovulation, two alternatives offer significantly better protection.
Ella (ulipristal acetate) is a prescription emergency contraceptive pill that works differently from Plan B. It’s a progesterone receptor modulator, meaning it can delay ovulation even after the hormonal surge has begun, a point at which Plan B is no longer effective. Ella is more effective than Plan B at one, three, and five days after unprotected sex. It requires a prescription in the US, but some telehealth services can provide one quickly.
The copper IUD is the most effective form of emergency contraception regardless of where you are in your cycle. Inserted by a healthcare provider within five days of unprotected sex (and in some cases beyond that window), it has a failure rate of just 0.12% across studies. Unlike hormonal methods, the copper IUD works by creating an environment that’s inhospitable to sperm and fertilization, so it doesn’t depend on blocking ovulation. It also doubles as long-term birth control for up to 10 years if you choose to keep it in place.
If timing and ovulation are uncertain, the copper IUD removes the guesswork entirely. Ella is the next best option when the IUD isn’t accessible. Plan B remains a reasonable choice when taken early in the fertile window, well before ovulation, but it is the least reliable of the three when ovulation is imminent or already underway.