After taking birth control pills without withdrawal bleeding, may be in the second half of menstruation to start taking drugs or contraceptive failure and other reasons. 1. The time of taking the pill: when you start taking the pill in the second half of your menstrual period, you will have ovulation and the formation of the corpus luteum, and the endometrium will have thickened under the action of progesterone secreted by the corpus luteum, so you will not have withdrawal bleeding after taking the pill, which is a normal phenomenon and can be observed temporarily. 2. Contraceptive failure: generally after taking the pill, there will be irregular vaginal bleeding or menstrual disorders and other symptoms, but when menstruation is delayed for more than 1 week, there is a possibility of contraceptive failure, and it is necessary to go to the hospital in a timely manner to carry out blood or urine HCG (human chorionic gonadotropin) test to rule out the possibility of pregnancy. After taking birth control pills without withdrawal bleeding if there is no obvious discomfort can first wait patiently, when menstruation overdue for more than 1 week should go to the hospital for examination in order to rule out the possibility of pregnancy.