Taking birth control pills leads to early menstruation, which usually occurs when taking emergency contraceptive pills. The main ingredient of emergency contraceptive pills is large doses of progesterone, and these exogenous progesterone will act on the endometrium when it enters the body, causing the endometrium to change from the proliferative phase to the secretory phase. When the progesterone in the body is completely metabolized and discharged from the body, the endometrial tissue in the secretory phase will undergo necrosis and shedding due to the rapid decline in progesterone levels, leading to withdrawal bleeding. If the time of taking the emergency contraceptive pill is before the 20th day of the menstrual cycle, then it will lead to early menstruation. If the emergency contraceptive pill is taken after the 20th day of the menstrual cycle, the onset of menstruation is on time or delayed. If there is no vaginal bleeding after one week of taking the pill, the possibility of pregnancy should be highly considered.