There are two main cases of withdrawal bleeding when taking birth control pills: first, withdrawal bleeding caused by emergency contraceptives. Emergency contraceptives are highly effective estrogens, and the patient’s blood progesterone and HCG levels rise rapidly after progesterone use, and then the hormone levels fall rapidly after metabolism, and this fluctuation in hormone levels can cause endometrial exfoliation and bleeding, called withdrawal bleeding. The amount of withdrawal bleeding is relatively small and can be cleaned within a week. If the bleeding amount is large or the bleeding time is long, it is considered to be irregular vaginal bleeding caused by endocrine disorder due to the contraceptive pill, which needs to be treated in time. Second, the use of compound short-acting contraceptive pills caused by withdrawal bleeding, compound short-acting contraceptive pills is estrogen and progestin complex preparation progestin-based, need to use 21 days, after stopping the drug patients will appear withdrawal bleeding, this withdrawal bleeding has a cycle, as regular monthly menstruation, will not affect the patient, do not need to intervene.