No, Plan B does not work if you are already pregnant and it does not affect an existing pregnancy.
Many people reach for Plan B after unprotected sex or a broken condom and then start to worry about one big question: does plan b work if you are already pregnant? Clear facts matter here, because this pill is about timing, not about ending a pregnancy.
Does Plan B Work If You Are Already Pregnant? Detailed Answer
Medical agencies that review Plan B are clear on this point. Plan B, which contains the hormone levonorgestrel, can lower the chance of pregnancy only when taken before a pregnancy has started. Once an egg has implanted in the uterus, the pill no longer has an effect on that pregnancy.
The United States Food and Drug Administration explains on the Plan B One-Step label that the pill will not work for someone who is already pregnant and will not affect an existing pregnancy.
In plain terms, if a pregnancy test is already positive, or if implantation has already happened, Plan B will not end that pregnancy, change the course of the pregnancy, or cause a miscarriage.
| Scenario | What Is Going On Biologically | Can Plan B Help? |
|---|---|---|
| Within 24 hours after unprotected sex | Ovulation may not have happened yet | Yes, this is when it works best |
| 24 to 72 hours after unprotected sex | Ovulation may be close or already past | Yes, but protection drops as time passes |
| 72 to 120 hours after unprotected sex | Fertilization or implantation may already be underway | Effect is lower; another method may work better |
| After ovulation but before implantation | Fertilization may already have happened | Plan B has little or no effect at this point |
| After implantation in the uterus | Pregnancy has started by medical definition | No, Plan B does not work here |
| Positive pregnancy test before taking Plan B | Hormone from the pregnancy is already present | No, it will not change the test or the pregnancy |
| Suspected ectopic (tubal) pregnancy | Fertilized egg grows outside the uterus | No, and this situation needs urgent medical care |
What “Already Pregnant” Means With Emergency Contraception
The timing words used in everyday speech often differ from medical terms. In daily language, a person might say they are pregnant as soon as they have unprotected sex or miss a period. Health professionals use a narrower definition tied to implantation.
Pregnancy in medical language starts when a fertilized egg attaches to the lining of the uterus. Before that moment the body does not class someone as pregnant, and home tests stay negative because hCG levels are still low.
Plan B works earlier in this timeline. It mainly delays or blocks ovulation, which is the release of an egg from the ovary. If there is no egg in the fallopian tube when sperm arrive, fertilization cannot happen, so there is no pregnancy to begin with.
Because of this, Plan B fits into the window between sex and implantation. It is meant for use soon after a risk, before there is a pregnancy. Once implantation has taken place, the pill does not stop the process and does not end a pregnancy.
Taking Plan B When You Are Already Pregnant
Plenty of people take Plan B and later learn that they were already pregnant when they swallowed the pill. That might happen because ovulation came earlier than expected, because bleeding was lighter and mistaken for a period, or because cycles are irregular.
Research on levonorgestrel used for emergency contraception shows no link between taking Plan B during an existing pregnancy and birth defects or pregnancy loss. The hormone dose is small compared with the level of hormones made during pregnancy itself.
If you took Plan B and then found out you were pregnant, there is usually no need to repeat the dose or expect harm from the pill. Your next steps are the same as with any early pregnancy: confirm the timing and talk with a health professional about care and options.
One warning sign needs fast care. If you have sharp one-sided pelvic pain, dizziness, or shoulder pain along with positive pregnancy testing, seek urgent help in case of ectopic pregnancy, because that condition can become life threatening and Plan B will not treat it.
How Plan B Works In The Body
Plan B One-Step and similar products contain a single 1.5 milligram dose of levonorgestrel, which is a synthetic form of the hormone progesterone. This same hormone is used in many regular birth control pills, only in a different schedule and dose.
When taken as emergency contraception, levonorgestrel mainly delays or blocks ovulation. If the pill is taken before the body releases an egg, the hormone can push that release later, past the time when sperm are able to fertilize the egg.
Large reviews of emergency contraception methods show that levonorgestrel pills do not interrupt an implanted pregnancy and do not raise the rate of miscarriage compared with people who did not take the pill. The primary role is prevention before pregnancy starts, not disruption once it exists.
Plan B works best when taken as soon as possible after unprotected sex. The label states that it is meant for use within 72 hours. Some data suggest a small effect up to 120 hours, but protection drops and other methods can work better late in that window.
Comparison With Other Emergency Contraception Methods
The American College of Obstetricians and Gynecologists describes emergency contraception choices, including levonorgestrel pills like Plan B, ulipristal acetate pills, and copper IUDs, and notes that they all work before pregnancy is established.
When you compare these options, timing, access, and health history all matter. Plan B is on the shelf in many regions, so it may be the fastest choice. Ulipristal acetate and copper IUDs often need a prescription or clinic visit but give stronger protection close to ovulation or several days after sex.
| Emergency Method | Time Window After Sex | Works If Already Pregnant? |
|---|---|---|
| Plan B (levonorgestrel pill) | Best within 72 hours, some effect up to 120 hours | No, it does not affect an existing pregnancy |
| Ulipristal acetate pill (ella) | Up to 120 hours with steady effect across that window | No, it does not end a pregnancy |
| Copper IUD used as emergency contraception | Up to 120 hours after sex, sometimes longer based on ovulation timing | No, it prevents new pregnancies but does not end one |
| Levonorgestrel IUD placed after sex | Up to 120 hours in some protocols | No, it prevents new pregnancies |
| Regular birth control pill packs | Not recommended as emergency contraception without medical guidance | No, standard packs do not end a pregnancy |
When To Take A Pregnancy Test After Using Plan B
After Plan B, many people watch their bodies closely and feel nervous about every cramp or spot of bleeding. The pill can shift the next period so that it arrives a few days earlier or later than expected, and flow can be lighter or heavier than usual.
A home pregnancy test is usually reliable about three weeks after the unprotected sex that led you to take Plan B, or at least one week after a missed period. Testing earlier may give a false negative result because hCG levels need time to rise into the detectable range.
If the test is negative and a full period arrives, pregnancy is strongly unlikely. If the test is negative but no period comes after three weeks, repeat testing or speak with a clinician, as delayed ovulation can shift the timing.
If the test is positive after you used Plan B, the pill did not work in that case. The pregnancy started after sex and the pill could not change it once implantation took place. Your options then depend on local rules and your own health and values, so meet with a trusted clinician or clinic.
Side Effects And Safety Points
Most people tolerate Plan B without serious problems. The most common short term effects are nausea, tiredness, breast soreness, headache, or mild abdominal cramps. These effects usually fade within a day or two.
Vomiting within two hours of taking the pill may mean the dose did not absorb fully. In that case, medical contact is sensible, since you may need another dose or another method.
People with a history of allergy to levonorgestrel or to any ingredient in the pill should avoid Plan B. Those with severe liver disease, unexplained vaginal bleeding, or other complex health conditions should talk with a clinician about the safest form of emergency contraception for their situation.
Plan B does not protect against sexually transmitted infections, and it is not meant as a regular birth control method. Regular contraception methods such as daily pills, implants, IUDs, patches, or injections are more reliable for ongoing prevention.
Main Points About Plan B And Existing Pregnancy
does plan b work if you are already pregnant? Current evidence and drug labels give a consistent answer: no, it does not work once a pregnancy has started, and it does not harm that pregnancy.
Plan B works by delaying or stopping ovulation and is meant for use soon after unprotected sex, before implantation. If you find out that you were already pregnant when you took it, the pill will not end the pregnancy, and decisions about care then follow the same process as any early pregnancy.
For personalized guidance, especially if you have irregular cycles, health conditions, or limited access to care, reach out to a licensed doctor, midwife, or clinic in your area. This article gives general medical information and does not replace professional care.