Taking Plan B while already pregnant will not harm the pregnancy. The pill simply won’t work, because it prevents pregnancy by delaying ovulation, and once a fertilized egg has implanted, there is nothing for the drug to interrupt. Both the FDA and the World Health Organization state clearly that emergency contraception cannot end an established pregnancy or harm a developing embryo.
Why Plan B Has No Effect on an Existing Pregnancy
Plan B contains levonorgestrel, a synthetic form of progesterone. Its job is to delay or block the release of an egg from the ovary. If ovulation hasn’t happened yet, sperm have nothing to fertilize, and pregnancy is prevented. But if a fertilized egg has already attached to the uterine wall, levonorgestrel has no mechanism to dislodge it or interfere with its development.
This is a fundamentally different process from a medical abortion, which uses a different drug (mifepristone) that actively blocks the hormones sustaining a pregnancy. Research confirms that levonorgestrel “does not prevent embryo implantation and therefore cannot be labeled as abortifacient.” The FDA’s own label for Plan B One-Step says it plainly: “Plan B One-Step will not work if you are already pregnant and will not affect an existing pregnancy.”
No Increased Risk of Birth Defects or Miscarriage
Several studies have looked specifically at what happens when women take levonorgestrel during the cycle they conceive or in early pregnancy. A CDC safety review examined a prospective study comparing 332 women who took emergency contraception during their conception cycle with 332 women who did not. There were no differences in first-trimester miscarriages or birth defects between the two groups, whether measured by ultrasound or at delivery.
A smaller retrospective study of 36 pregnant women who took emergency contraception in the first trimester found no significant differences in miscarriage or stillbirth rates compared with 80 unexposed women. Among pregnancies that continued after emergency contraception failure, normal outcomes were reported for all that were followed to delivery. Miscarriage rates were consistent with what you’d expect in the general population.
No Higher Risk of Ectopic Pregnancy
You may have seen claims that Plan B increases the chance of an ectopic pregnancy, where a fertilized egg implants outside the uterus, usually in a fallopian tube. There’s a theoretical explanation for this: levonorgestrel can slow the movement of tiny hair-like structures in the fallopian tubes that help transport an egg. In theory, this could delay an egg’s journey enough for it to implant in the wrong place.
In practice, large studies have not confirmed this risk. A meta-analysis of 136 studies found that the ectopic pregnancy rate among women who used levonorgestrel was 1.6%, which is actually lower than the general population rate of about 2%. Well-powered studies designed to detect this specific risk consistently show no increased threat of ectopic pregnancy after emergency contraception use.
Why You Might Not Know You’re Pregnant
Most people who take Plan B while unknowingly pregnant do so because they don’t yet have any signs of pregnancy. If you had unprotected sex more than a few days before taking the pill, fertilization and implantation may have already occurred. Plan B is most effective when taken within 72 hours of intercourse, and its effectiveness drops significantly after that window. If you took it late, or if you had already ovulated and conceived from an earlier encounter, the pill wouldn’t have prevented pregnancy.
Plan B can also cause symptoms that mimic early pregnancy: nausea, breast tenderness, fatigue, and changes to your next period. Your period may come early, late, or be heavier or lighter than usual. These side effects can make it hard to tell whether the pill worked or whether you’re pregnant.
When to Take a Pregnancy Test
If your period is more than a week late after taking Plan B, a pregnancy test is a reasonable next step. Home pregnancy tests detect a hormone that the body only produces after a fertilized egg implants in the uterus. This takes time. A test won’t pick up a pregnancy until at least 10 days after conception, and waiting a full two weeks reduces your chance of getting a false negative.
If you get a positive result, the reassuring takeaway is this: the Plan B you took will not have affected the pregnancy. Every study examining this question has reached the same conclusion. The pregnancy will proceed as though the pill was never taken.