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 that process is already underway, the pill has essentially missed its window. In clinical studies, women who took Plan B on the day of ovulation or after became pregnant at the same rate as women who used no contraception at all.
Why Plan B Doesn’t Work After Ovulation
Plan B contains a synthetic hormone called levonorgestrel. Its main job is to delay or suppress the hormonal surge that triggers the release of an egg. If you take it early enough in your cycle, before that surge peaks, it can push ovulation back by several days. By then, any sperm from unprotected sex have died off (sperm survive about 3 to 5 days inside the reproductive tract), and fertilization never happens.
The problem is timing. Once your body’s hormonal surge has already peaked and the egg has been released, levonorgestrel can’t reverse that. In pooled data from three randomized trials, Plan B delayed ovulation in only about 15% of cycles when given during the late fertile window, which was statistically no different from a placebo. The drug simply doesn’t have a meaningful effect once the ovulation process is in motion.
How to Tell If You’ve Already Ovulated
Most people can’t pinpoint the exact moment of ovulation without tracking tools, but there are practical signs that can help you estimate where you are in your cycle. The clearest real-time indicator is a home urine test that detects the luteinizing hormone (LH) surge. These monitors correctly identified ovulation about 91% of the time during the two peak fertility days.
Cervical mucus changes are another signal. In the days leading up to ovulation, mucus becomes clear, stretchy, and slippery, similar to raw egg whites. This “peak” mucus pattern lines up with ovulation within one day about 78% of the time. Basal body temperature (the slight rise you see after ovulation) only confirms ovulation after the fact, so it’s less useful for making a real-time decision about emergency contraception.
If you weren’t tracking your cycle, a rough estimate based on cycle length can help. Ovulation typically occurs about 14 days before your next expected period. So in a 28-day cycle, that’s around day 14. But cycles vary, and stress, illness, or irregular periods can shift ovulation by days.
A More Effective Option Near Ovulation
If you suspect you’re close to ovulating or already in the process, a prescription emergency contraceptive called ella (ulipristal acetate) is significantly more effective than Plan B at this stage. In the same pooled trials, ella delayed ovulation for at least five days in 59% of cycles when given during the late fertile window. Even when taken during the LH surge itself, ella prevented the egg’s release 79% of the time, compared to just 14% for Plan B and 10% for a placebo.
That said, no oral emergency contraceptive worked when taken on the actual day the LH surge peaked. So ella is better than Plan B near ovulation, but neither pill is reliable once ovulation is fully complete.
The Copper IUD Is the Most Reliable Backup
The most effective form of emergency contraception at any point in the cycle, including after ovulation, is a copper IUD. Unlike oral options that work by blocking ovulation, a copper IUD creates an environment in the uterus that prevents a fertilized egg from implanting. It can be inserted up to five days after unprotected sex, or up to five days after your estimated ovulation date.
The numbers are striking: across 42 studies, the pregnancy rate with a copper IUD used as emergency contraception was 0.09%, roughly ten times more effective than any oral option. It also doubles as ongoing birth control for up to 10 years, though it can be removed at any time if you change your mind. Getting one placed does require a clinic visit, so calling your provider or a local health center quickly is key if you’re considering this route.
Body Weight Can Further Reduce Plan B’s Effectiveness
Beyond timing, your weight plays a role in how well Plan B works even under ideal circumstances. Research shows that levonorgestrel’s effectiveness begins to decline at around 70 kg (about 154 pounds) and drops to essentially no efficacy at 80 kg (about 176 pounds) and above. People with a BMI of 30 or higher had more than four times the risk of pregnancy compared to those with a BMI under 25 when using Plan B.
This matters because if you’re both near ovulation and above these weight thresholds, your chances of Plan B working are especially low. UK clinical guidelines have suggested a double dose of levonorgestrel for people at higher weights, but the evidence behind this is limited. Ella or a copper IUD are generally more reliable choices in this situation.
What to Expect After Taking Plan B
Regardless of whether Plan B prevents pregnancy, it will likely affect your next period. How it shifts your cycle depends on when you took it. Women who took it before ovulation saw their period arrive about one day early. Those who took it after ovulation experienced a delay of close to two days. Interestingly, women who took it right around the time of expected ovulation saw no significant change in cycle length.
Overall, about one in five women had their cycle shorten by two or more days, and about one in four had it lengthen by two or more days. Some spotting between periods is also common. If your period is more than a week late, a pregnancy test is a reasonable next step.