Plan B, also known as the morning-after pill, is a form of emergency contraception containing a high dose of the hormone levonorgestrel. It is a valuable tool for preventing pregnancy after unprotected sex, but it is not infallible. For those who find themselves pregnant after taking Plan B, several factors may have contributed to its failure. Understanding these reasons is important for both addressing the current situation and making informed choices about future contraception.
How Plan B Works (and Doesn't)
At its core, Plan B works by preventing or delaying ovulation. It releases a concentrated dose of the same progestin hormone used in many daily birth control pills, which inhibits the release of an egg from the ovary. If an egg is not released, there is nothing for sperm to fertilize, thus preventing pregnancy.
However, this mechanism has a critical limitation: it is only effective before ovulation has already occurred. Once the ovary has released an egg, Plan B cannot stop it from being fertilized by sperm, which can live inside the body for up to five days. The timing of intercourse in relation to the fertile window is a key determinant of the medication's success.
Key Factors That Can Lead to Plan B Failure
Timing is Everything
Plan B is most effective the sooner it is taken after unprotected sex. While it can be taken up to 72 hours (three days) later, its effectiveness diminishes with each passing hour. The manufacturer states it is most effective when taken within the first 24 hours. This is because it gives the medication the best chance to delay ovulation before it happens. If taken too late, particularly during the fertile window, the risk of pregnancy increases significantly.
Body Weight Considerations
Research suggests that Plan B may be less effective for individuals with a higher body weight or Body Mass Index (BMI). Specifically, studies indicate a reduced effectiveness in people weighing over 165 pounds or with a BMI over 30. One theory is that a higher body mass may cause the medication to be less concentrated in the bloodstream, reducing its hormonal effect. For individuals concerned about this, alternative emergency contraception options like Ella (ulipristal acetate) or a copper IUD may be more effective.
Drug and Herbal Interactions
Certain medications and herbal supplements can interfere with the way the body processes the hormones in Plan B, making it less effective. This can lead to a concentration of the drug that is too low to prevent ovulation. Common interacting substances include:
- The herbal supplement St. John's wort
- Certain antibiotics
- Some anti-seizure medications, such as carbamazepine and topiramate
- Select HIV medications
It is crucial to inform your doctor or pharmacist of all medications and supplements you are taking if you need emergency contraception.
Vomiting After Taking the Pill
If a person vomits within two to three hours of taking Plan B, the body may not have had enough time to absorb the medication. In this event, a healthcare professional should be consulted to determine if a repeat dose is necessary.
Repeat Acts of Unprotected Sex
Plan B provides protection for a single instance of unprotected sex. It does not offer ongoing contraception. If you have unprotected sex again after taking the pill, you are at risk of pregnancy. The pill's effects wear off relatively quickly, and your menstrual cycle will continue. You must use another method of contraception for any subsequent sexual activity.
What to Do if Plan B Fails
If you believe Plan B has failed, the only way to be certain is to take a pregnancy test. If your period is more than a week late after taking the pill, or if you experience early pregnancy symptoms, it is recommended to take a test.
Once pregnancy is confirmed, it is important to consult with a healthcare provider to discuss your options. Plan B and other emergency contraceptive pills will not harm an existing pregnancy or the fetus. Your healthcare provider can discuss prenatal care, adoption, or abortion options with you, based on your circumstances and local laws.
Comparison of Emergency Contraception Options
| Feature | Plan B (Levonorgestrel) | Ella (Ulipristal Acetate) | Copper IUD |
|---|---|---|---|
| Active Ingredient | Levonorgestrel (synthetic progestin) | Ulipristal acetate | Copper |
| Time to Take | Within 72 hours (3 days) of unprotected sex; sooner is better | Within 120 hours (5 days) of unprotected sex | Inserted within 120 hours (5 days) of unprotected sex |
| Primary Mechanism | Delays or prevents ovulation | Delays ovulation even closer to the time it occurs | Prevents fertilization; creates toxic environment for sperm |
| Effectiveness | Approx. 89% within 72 hours; higher if taken sooner | Approx. 95% effective | >99% effective |
| Weight Limitations | Less effective for those over 165 lbs or BMI >30 | Less effective for those over 195 lbs or BMI >35 | No weight limitations |
| Availability | Over-the-counter (OTC) | Prescription required | Requires a healthcare provider to insert |
| Ongoing Contraception | No. Only provides emergency protection for a single incident. | No. Only provides emergency protection for a single incident. | Yes, provides long-term contraception for up to 10 years |
Conclusion
For anyone asking, "Why did I get pregnant on Plan B?", the answer can be multifaceted. The most common reasons relate to timing, as Plan B is not effective if taken after ovulation has already occurred. Other contributing factors include the individual's body weight, drug interactions, and repeat unprotected sex after taking the pill. It is not a guaranteed solution, but rather a tool to significantly reduce the risk of pregnancy. If failure does occur, it's essential to consult with a healthcare professional to understand all available options and ensure your reproductive health is managed responsibly. The most effective emergency contraception is a copper IUD, which also offers long-term prevention.
For more information on emergency contraception options and resources, you can visit the Planned Parenthood website.
Medical Disclaimer
The information provided in this article is for educational purposes only and should not be considered medical advice. Always consult with a qualified healthcare provider for diagnosis and treatment of any health condition, and before making any decisions related to your medical care or treatment.
References
- CVS. (2025, June 27). Plan B One-Step Emergency Contraceptive Tablet. https://www.cvs.com/shop/plan-b-one-step-emergency-contraceptive-tablet-prodid-876669
- Plan B One-Step®. (n.d.). How Plan B Works. https://www.planbonestep.com/how-plan-b-works/
- SingleCare. (2023, July 24). How effective is Plan B?. https://www.singlecare.com/blog/how-effective-is-plan-b/
- Medical News Today. (2023, February 1). Signs that Plan B did not work. https://www.medicalnewstoday.com/articles/signs-that-plan-b-didnt-work
- Healthline. (2025, March 12). Why Plan B Doesn't Work During Ovulation and Alternatives. https://www.healthline.com/health/healthy-sex/does-plan-b-work-during-ovulation
- Hey Jane. (2025, July 7). Plan B myths. https://www.heyjane.com/articles/plan-b-myths
- Fort Worth Pharmacy. (n.d.). What is The Important Link Between Body Weight and Emergency Contraception Solutions? https://www.fwpharm.com/blog/what-is-the-important-link-between-body-weight-and-emergency-contraception-solutions
- Planned Parenthood of Michigan. (n.d.). Emergency Contraception. https://www.plannedparenthood.org/planned-parenthood-michigan/healthcare/emergency-contraception
- Healthline. (2017, February 2). Emergency Contraception: What to Do Afterward. https://www.healthline.com/health/birth-control/what-to-do-afterward
- Natural Cycles. (2025, August 22). Signs that Plan B didn't work: Why it happens & what to do. https://www.naturalcycles.com/cyclematters/signs-that-plan-b-did-not-work
