Signs that Plan B did not work may include early symptoms of implantation or pregnancy. Examples of these symptoms include light bleeding, tiredness, nausea, dizziness, or a rapid heartbeat.

Implantation occurs when a fertilized egg embeds into the lining of the uterus. It typically occurs 7 days after sexual intercourse, but not everyone experiences symptoms this early. Additionally, many other things could cause these symptoms. As a result, it is impossible to confirm a pregnancy based on early symptoms alone.

A person can be sure by taking a home pregnancy test. People can typically get an accurate result from a pregnancy test anytime after their missed period or 21 days after having sex.

Read on to learn the signs Plan B did not work.

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The most reliable signs that Plan B did not work are a missed period and a positive pregnancy test.

While some people may also experience early symptoms of pregnancy, not everyone does, which means they are not a reliable indication that emergency birth control has failed.

If a person does experience early signs of pregnancy, they may have:

However, the presence of these symptoms does not necessarily mean a person is pregnant, as other conditions can cause them. Similarly, their absence does not mean that a person is not pregnant.

People cannot confirm that Plan B has not worked until they take a pregnancy test.

At implantation, the body begins producing a pregnancy hormone known as human chorionic gonadotropin (hCG). This is the hormone that a home pregnancy test detects.

Implantation typically occurs around 7 days after the fertilization of an egg. At this point, hCG levels start to rise quickly.

In most cases, a person can get a positive pregnancy test by the time they miss their period, or if they are not sure when their period is due, 21 days after having sex.

Plan B is highly effective if a person uses it within 72 hours after sexual intercourse without a barrier method. The earlier a person uses it, the more effective it is.

Plan B suppresses ovulation, which can prevent sperm from fertilizing an egg. Therefore, taking Plan B as early as possible gives the medication the best chance of working. Plan B also makes it more difficult for a pregnancy to implant in the uterus if a person does ovulate.

In tests by the manufacturer, Plan B One-Step was effective at preventing 84% of pregnancies. A typical pregnancy rate among the 1,198 females included in the study group would be around 8%, but Plan B One-Step reduced this rate to about 1%.

Some reasons Plan B might fail include:

  • Ovulation: Plan B works primarily by delaying or preventing ovulation. If a person ovulates before taking it, they may still get pregnant.
  • Weight: Some studies suggest that Plan B works less well in people with a body mass index (BMI) of over 30. There is no official Plan B weight limit, and the drug can still work in people with larger bodies, but it may work less effectively.
  • Vomiting: If a person vomits within 2 hours of taking the drug, they need to repeat the dose.
  • Drug interactions: A small number of medications and supplements may affect the efficacy of Plan B. They include St. John’s wort, barbiturates, and topiramate. A person should discuss all medications they take with a doctor or pharmacist before using Plan B.
  • Timing: Plan B works best when a person takes it as soon as possible after sex without a barrier method. Waiting longer than 72 hours will dramatically decrease its effectiveness.
  • Pregnancy: Plan B is not an abortion pill and will not induce an abortion in a person who is already pregnant.

Plan B alternatives may be a more suitable option for some groups. For example, a 2019 study found that ulipristal may be more effective in people with a BMI over 25 or who weigh over 165 pounds (74.8 kilograms).

There is no research on whether specific food affects the efficacy of Plan B.

Pregnancy is not guaranteed after sex without a barrier method, even if a person does not use Plan B. Research from 2022 that has yet to be published found that spontaneous sex without a barrier method has a pregnancy rate of around 18%.

Timed intercourse, which means timing intercourse for when a person is most likely to be ovulating, has a pregnancy rate of 20–28%.

Emergency birth control makes pregnancy less likely. A 2019 study of 1,000 females in Japan who sought emergency birth control found that just 16 of the participants, or 2.4%, were pregnant at follow-up.

If a person thinks Plan B might not have worked, they should try taking a home pregnancy test on the day their period is late or 3 weeks after they had sex if they are not sure when their period is due.

Alternatively, people can seek testing from a doctor. People who cannot access healthcare may be able to get a pregnancy test at their county department of health, at a discount clinic, or at a local Planned Parenthood.

When doing this, it is important to be aware of “crisis pregnancy centers”. These are facilities that claim to offer free support for people with unplanned pregnancies, and they may offer free pregnancy tests. But these facilities do not give impartial, unbiased advice. Additionally, they may have no medical staff.

Crisis pregnancy centers often appear close to Planned Parenthood health centers. They may also have similar names. Use Planned Parenthood’s official Find a Health Center tool to find the exact address.

If the test is positive, a person should speak with a doctor or midwife as soon as possible to determine how far into the pregnancy they are. The earlier a person does this, the more options they will have when deciding the next steps.

If a person wants to continue their pregnancy or needs more time to decide, they should begin taking prenatal vitamins and avoid alcohol. If a person smokes or is often around secondhand smoke, they should try to reduce their exposure.

Learn more about family planning options in abortion-restricted states.

Early pregnancy symptoms, such as fatigue, nausea, or mood swings, may suggest that Plan B did not work. However, these symptoms can happen for many reasons, so they are not reliable. A pregnancy test is the only way to confirm Plan B did not work.

It is usually possible to confirm a pregnancy with a home test or a test at a doctor’s office anytime after the first day of a missed period.

Plan B usually works, and the odds of pregnancy from a single instance of sex without a condom or other barrier method followed by emergency birth control are low. If a person is in any doubt, they should seek out testing as soon as possible so they can discuss their options with a health professional.