Why doesn't menstruation come after taking emergency contraception?

Patient's question:

At this point, my period has already started. I will definitely not get pregnant, what if I do? I'm a bit overweight, does this affect the efficacy of levonorgestrel enteric-coated tablets? Medical history and treatment outcomes: None

Doctor's answer:

Hello, according to your description, it is recommended that you do not feel nervous. Delayed menstruation after taking emergency contraception pills is a normal phenomenon, and this is a common side effect of emergency contraception pills. You must not be nervous.
Suggestions: If your menstruation is delayed by a few days, you can use a pregnancy test kit in the morning. Generally, taking emergency contraception pills on time will not result in pregnancy. If the test is negative, you no longer need to be nervous. Breast tenderness cannot be ruled out as a symptom of menstruation.