Plan B does not work after ovulation. Its only proven mechanism is preventing or delaying the release of an egg, so once that egg is already out, the drug has no meaningful effect on pregnancy rates. In studies tracking women with blood tests and ultrasounds to confirm exactly where they were in their cycles, those who took Plan B on the day of ovulation or after became pregnant at the same rate as if they had taken nothing at all.
How Plan B Actually Works
Plan B contains a single high dose of levonorgestrel, a synthetic hormone. It works by interrupting the hormonal chain reaction that triggers ovulation. Specifically, it suppresses or delays the surge of luteinizing hormone (LH) that tells your ovary to release an egg. If that surge hasn’t started yet, Plan B can effectively shut it down and prevent ovulation from happening. No egg, no fertilization, no pregnancy.
This is the only confirmed mechanism. A series of studies reviewed by the FDA found no evidence that levonorgestrel interferes with fertilization or prevents a fertilized egg from implanting in the uterus. The FDA has formally classified it as a nonabortifacient, meaning it does not interrupt a pregnancy that has already begun at any stage.
What the Numbers Show
The clearest data comes from studies that used blood hormone levels and ultrasounds to pinpoint each woman’s exact cycle day, rather than estimating based on period dates. In a combined total of 492 women who took Plan B for emergency contraception, the results split sharply based on timing:
- Taken before ovulation: Zero pregnancies occurred out of 20 expected. That’s essentially 100% effectiveness.
- Taken on the day of ovulation or after: 11 pregnancies occurred, and 11 to 12 were expected. That’s the same pregnancy rate as using no contraception at all.
A separate analysis of 148 women who had intercourse during their fertile window found similar results. Among the 103 who took Plan B before ovulation, not a single pregnancy occurred despite 16 being statistically expected. Among the 45 who took it on ovulation day or later, 8 pregnancies occurred out of 8.7 expected. The drug made no difference.
These findings also mean Plan B’s overall effectiveness has been overestimated in older studies. Those earlier numbers were based on estimated cycle dates rather than confirmed hormone levels, which introduced significant errors. When timing is measured precisely, Plan B is highly effective before ovulation and essentially useless after it.
The LH Surge Is the Cutoff Point
The practical cutoff isn’t ovulation itself. It’s the LH surge that happens roughly 24 to 36 hours before the egg is released. Once that surge has started, Plan B can no longer suppress it effectively enough to prevent ovulation. This means there’s a window of several hours before the egg is technically released where Plan B is already too late to help.
The strength and duration of the LH surge also matter for natural fertility. Research shows that a strong, two-day surge results in pregnancy rates around 23%, while a weak, one-day surge drops that to about 5.6%. Plan B works by preventing the surge entirely, not by weakening it partway through.
How to Tell If You’ve Already Ovulated
This is the frustrating part: most people can’t know with certainty whether they’ve ovulated without blood work or an ultrasound. But there are clues that can help you estimate where you are in your cycle.
If you track your cycles and they’re fairly regular, ovulation typically happens about 14 days before your next expected period (not 14 days after your last one, which is a common misunderstanding). So in a 28-day cycle, that’s around day 14. In a 32-day cycle, it’s closer to day 18. Cervical mucus that looks and feels like raw egg whites, clear and stretchy, is a sign you’re in or approaching your fertile window. A sustained rise in basal body temperature confirms ovulation has already happened, but only if you’ve been tracking it daily before that point.
Over-the-counter ovulation predictor kits detect the LH surge in your urine. If you’ve recently gotten a positive result on one of these, the surge has likely already begun, which means Plan B may no longer be able to block ovulation.
Ella Works Later in the Cycle
If you think you may have already started your LH surge or are close to ovulation, a different emergency contraceptive called ella (ulipristal acetate) is a better option. Unlike Plan B, ella can disrupt ovulation even after the LH surge has begun. This is the main reason ella is more effective overall: it covers a wider window of the cycle.
Ella requires a prescription in the United States, which can be a barrier when time matters. But it remains effective for up to five days after unprotected sex, and its advantage over Plan B grows larger the closer you are to ovulation. If you’re midcycle and concerned about timing, ella is worth pursuing.
The Copper IUD as Emergency Contraception
The most effective form of emergency contraception is a copper IUD inserted within five days of unprotected sex. Unlike hormonal options, the copper IUD works primarily by creating an environment that’s toxic to sperm, and it can also prevent implantation. This means it remains effective regardless of where you are in your cycle, including after ovulation. It reduces the risk of pregnancy by more than 99% when used as emergency contraception, and it then continues working as long-term birth control for up to 10 to 12 years.
Getting one placed requires a clinic visit, which isn’t always possible on short notice. But if you can access it within the five-day window, it’s the single most reliable option available, especially when ovulation timing is uncertain.
What This Means for Your Decision
If you had unprotected sex in the first half of your cycle, well before you expect to ovulate, Plan B is a strong choice and works very well. If you’re midcycle, near ovulation, or unsure of your timing, ella or a copper IUD gives you meaningfully better protection. The key variable isn’t how quickly you take emergency contraception after sex. It’s where you are in your menstrual cycle when you take it.
Plan B is available without a prescription and works fast when the timing is right. But its limitations are real and sharply defined. Once ovulation has occurred, or even once the hormonal process leading to ovulation is underway, it provides no additional protection against pregnancy.