Menstruation after taking birth control pills is due to withdrawal bleeding caused by birth control pills. The main ingredients of short-acting contraceptive pills are estrogen and progestin. When you take short-acting contraceptive pills regularly, breakthrough bleeding may occur for a short period of time when you first start taking them, and the bleeding is very small and can stop on its own, unlike your usual period. This symptom will disappear after 2-3 cycles of the pill. After taking a cycle of the short-acting pill, there will be more withdrawal bleeding for about 3-5 days after stopping the pill, equivalent to the usual amount of menstruation, which is a menstrual flow. After taking the emergency contraceptive pill, if the time of taking the pill is in the second half of the menstrual cycle, there will be a more obvious withdrawal bleeding, the amount of bleeding is basically the same as the usual amount of menstruation, which can be regarded as a menstrual flow, in this case generally indicates the success of contraception.