Why Does "Plan B" Cause Spotting?

The emergency contraception Plan B One-Step causes spotting because the levonorgestrel it contains may alter the lining of the uterus to prevent a fertilized egg from implanting, states WebMD. Plan B may also cause menstrual changes, prevent or delay ovulation and prevent an egg from becoming fertilized.

Plan B can prevent pregnancy if taken 72 hours after a woman has had unprotected sex. Plan B is not intended to be used regularly as contraception. It is not a birth control pill and it is not effective if taken prior to having unprotected sex, says WebMD.

Plan B is not the same as the abortion pill, RU-486, and it does not terminate a pregnancy, notes WebMD. Plan B is a back-up contraception women should take if their regular birth control method may have failed, or no birth control was used. Plan B may be appropriate to use if birth control was not used during sex, if a condom broke or fell off, if a diaphragm slipped out of place, or if birth control pills were skipped for more than two days, according to WebMD. Side effects of Plan B include abdominal pain, nausea, vomiting, changes in menstruation, fatigue, headache and dizziness.