The timing of menstruation after taking the pill is determined by the type of pill, which can be classified as long-acting, short-acting and emergency contraceptive pills. Long-acting contraceptives usually come after 1 month of taking the pill, short-acting contraceptives usually come 3-7 days after stopping the pill, and emergency contraceptives do not have a definite time of menstruation, which may be delayed. However, it is important to note that contraception is not always successful after taking the pill. In case of delayed menstruation, a blood and urine HCG test can be performed to determine if pregnancy is present. You can also have an ultrasound to check the thickness of the uterine lining to monitor the onset of menstruation.1. Long-acting contraceptives: such as levonorgestrel ethinyl estradiol tablets and other drugs, women can take the pill to achieve a longer contraceptive effect, without the need to take the pill frequently. It is possible to have menstruation 1 month after stopping the pill, but the adverse drug reactions are large and may lead to menstrual disorders or even amenorrhea. Because of the side effects have been less used in recent years; 2, short-acting contraceptives: most of the short-acting contraceptives in the 5th day of menstruation to start eating, eat 21 days in a row after 3-7 days to stop menstruation, and can play a month of contraceptive effect. The main component of short-acting contraceptive pills is estrogen and progestin, the main principle is to reduce ovulation and make the endometrium thinner, thus reducing the chances of fertilized eggs being laid, in order to achieve the purpose of contraception. Because it contains estrogen, it should be used with caution. Before using the pill, you must go to the hospital for a checkup to exclude contraindications to the use of the pill, such as the tendency to thrombosis, breast enlargement, uterine fibroids and other diseases. If you stop taking short-acting contraceptives and still do not have a period after 8 days, you must first rule out the possibility of pregnancy. Because the contraceptive effect of short-acting contraceptive pills, although better, but can not reach 100%; 3, emergency contraceptive pills: if it is taken a few days before the onset of menstruation, after stopping the pill usually causes withdrawal bleeding, and withdrawal bleeding is mainly caused by the shedding of the endometrium. It takes 3-5 days for the endometrium to repair before the bleeding stops. In this case, the time of menstrual flow may not be determined and delayed menstrual flow may occur. If the emergency contraceptive pill is taken during the second half of the menstrual period, the vast majority of them do not cause withdrawal bleeding, so the corpus luteum has already formed by then. The progesterone secreted by the corpus luteum is sufficient to support the endometrium from shedding, so even if the pill is stopped, there will be no withdrawal bleeding. In this case, menstruation will usually come at the original time of menstruation.