If you take Plan B after you’ve already ovulated, it is unlikely to prevent pregnancy. Plan B works almost entirely by delaying or blocking ovulation, so once an egg has already been released, the pill has little to no effect on your chances of conceiving. Clinical studies confirm this directly: when levonorgestrel (the active ingredient in Plan B) was given on the day of ovulation or after, the expected pregnancy rate was not reduced.
Why Plan B Doesn’t Work After Ovulation
Plan B prevents pregnancy by interrupting the hormonal chain of events that leads to ovulation. Specifically, it suppresses the surge of luteinizing hormone (LH) that triggers your ovary to release an egg. When you take it during the first half of your cycle, before that surge happens, it can delay or completely prevent egg release. No egg means no fertilization.
Once ovulation has already occurred, that window closes. The egg is already out, and Plan B has no mechanism to stop what happens next. The FDA reviewed the totality of evidence on this question and concluded that Plan B has no direct effect on fertilization or implantation. In other words, it won’t interfere with a fertilized egg attaching to the uterine lining. The strongest clinical data, from studies by Novikova (2007) and Noé (2011), showed that Plan B was effective only when given before ovulation day. From ovulation onward, it performed no better than taking nothing at all.
How to Tell If You’ve Already Ovulated
This is the tricky part. Most people don’t know exactly when they ovulate unless they’re actively tracking their cycle. Ovulation typically happens about 14 days before your next period starts, but this varies widely from person to person and cycle to cycle. A few signs can give you clues:
- Cervical mucus changes: In the days leading up to ovulation, cervical mucus becomes transparent, stretchy, and slippery, similar to raw egg white. After ovulation, it thickens, becomes sticky or creamy, and the wet sensation at the vulva disappears. If your mucus has already shifted from that egg-white texture to thick and dry, ovulation has likely passed.
- LH test strips: Over-the-counter ovulation predictor kits detect the LH surge in your urine. A positive result means ovulation is approaching (usually within 24 to 36 hours). If you got a positive strip a day or two ago, ovulation has probably already happened or is happening now.
- Basal body temperature: Your resting temperature rises slightly (about 0.5°F) after ovulation and stays elevated. If you’ve been charting and see that rise, the egg has already been released.
If you’re unsure where you are in your cycle, taking Plan B is still reasonable. It won’t cause harm, and there’s a chance you haven’t ovulated yet, especially if you’re in the first half of your cycle. But if you have strong evidence that ovulation already occurred, you should know that Plan B’s effectiveness drops dramatically.
Ella May Be a Better Option Near Ovulation
If you’re close to ovulation but haven’t quite gotten there yet, a prescription emergency contraceptive called ella (ulipristal acetate) has an advantage over Plan B. While Plan B loses its ability to delay ovulation once the LH surge has begun, ella can still suppress or delay ovulation even after the surge starts. This gives it a wider effective window in your cycle.
Ella also maintains consistent effectiveness for up to 120 hours (five days) after unprotected sex, while Plan B’s effectiveness drops significantly after 48 hours. In the 72-to-120-hour window specifically, one clinical trial found three pregnancies among women who received Plan B compared to zero among those who received ella. Ella requires a prescription, but some telehealth services can provide one quickly.
That said, if ovulation has fully completed, ella’s advantage narrows considerably. Its primary mechanism is also ovulation suppression.
The Copper IUD as a Post-Ovulation Option
The one form of emergency contraception that does work after ovulation is the copper IUD. When inserted within five days of unprotected sex, it prevents pregnancy with over 99% effectiveness regardless of where you are in your cycle. Unlike hormonal methods, the copper IUD creates an environment that is inhospitable to both sperm and a fertilized egg. If you’re confident you’ve already ovulated and preventing pregnancy is a priority, a copper IUD placed by a healthcare provider is the most reliable option available.
What Plan B Will (and Won’t) Do to Your Body
Even if Plan B doesn’t prevent pregnancy when taken after ovulation, it will still affect your cycle. The dose of synthetic progesterone can delay your next period by up to a week, which can create anxiety about whether or not you’re pregnant. You may also experience nausea, spotting, breast tenderness, headaches, or fatigue in the days after taking it. These are side effects of the hormone itself, not signs of pregnancy or proof that the pill worked.
If your period is more than a week late after taking Plan B, take a pregnancy test. The most reliable timing is three weeks after you took the pill, which gives enough time for pregnancy hormone levels to be detectable.
Body Weight Affects Effectiveness Too
Timing relative to ovulation is the biggest factor in whether Plan B works, but body weight also plays a role. If you weigh more than 165 pounds, levonorgestrel-based emergency contraception (Plan B and its generics) becomes significantly less effective. For people in this weight range, ella or the copper IUD are more reliable choices regardless of cycle timing.
The Bottom Line on Timing
Plan B is a before-ovulation pill. Its entire mechanism depends on preventing or delaying the release of an egg. The FDA has confirmed it does not affect fertilization, does not affect implantation, and does not terminate an existing pregnancy. If you took it after ovulation, it won’t increase your risk of harm to a potential pregnancy, but it also won’t reduce your chance of becoming pregnant. If you’re within five days of unprotected sex and suspect ovulation already happened, contacting a provider about a copper IUD gives you the strongest protection available.