After taking Plan B, three things may happen in a woman's body to prevent pregnancy. WebMD explains that Plan B One-Step may prevent or delay ovulation, interfere with fertilization of an egg or prevent implantation of a fertilized egg by changing the uterine lining. Plan B does not cause miscarriage or abortion, and it does not stop fetal development once a fertilized egg implants in the uterus.
According to WebMD, Plan B is a type of over-the-counter emergency contraception. Its dose is a single pill that contains 1.5 milligrams of levonorgestrel. The risk of pregnancy is reduced by up to 89 percent when Plan B is taken within 72 hours of unprotected sex. When taken within 24 hours, Plan B is about 95 percent effective.
Plan B is not as effective as regular contraception, and it should not be used as a woman's main form of birth control. A woman should not take Plan B if she knows or suspects she is pregnant, has a history of allergy to any ingredients in Plan B or has a history of abnormal vaginal bleeding that has not been evaluated by a doctor. Side effects that may occur after taking Plan B include nausea, abdominal pain, fatigue, headache, dizziness, vomiting and breast tenderness. The medication may also cause unexpected bleeding and menstrual changes, notes WebMD.