The emergency contraceptive pill is a highly effective progestin, and the contraceptive principle is to prevent ovulation of the ovaries. In addition, it can change the thickness of the endometrium, which causes abnormal thickening of the endometrium and prevents the fertilized egg from being laid, which is the contraceptive mechanism of the emergency contraceptive pill. If ovulation is prevented, it breaks the natural rule of the ovaries, which will have a certain effect on endocrine secretion, so there will be temporary disruption of menstruation or menstrual disorders. If the endometrium is abnormally thickened, if the emergency contraceptive pill is taken during the first half of the menstrual period, when the pill is stopped, the endometrium loses its exogenous progesterone support, and withdrawal bleeding occurs. If the emergency contraceptive pill is taken during the second half of the menstrual period, the endometrium will also appear abnormally thickened. However, during the second half of the period, due to the formation of the corpus luteum, the endometrium is not sensitive to the withdrawal of exogenous progesterone, so there is often no bleeding, and when the emergency contraceptive pill is taken during the second half of the menstrual period, menstruation will often come on time.