Oral emergency contraceptive pill, contraceptive failure is mainly due to the following reasons: first, due to the poor absorption of the drug, the body does not reach the effective concentration of the drug, and can not prevent ovulation of the ovaries, and can not make the endometrium abnormal thickening and prevent the fertilization of the egg, resulting in contraceptive failure. Second, the timing of oral medication is not right, such as not taking oral medication within 72 hours after intercourse, which causes the failure of contraception. Thirdly, due to individual differences, the effectiveness of emergency contraceptive pill itself is not 100%, in clinical practice, about 2%-3% of patients will become pregnant after taking emergency contraceptive pill.