Emergency contraceptive pills are useful to be taken orally within 72 hours after having sex without contraception. However, no medication is 100% effective in clinical contraception or treatment of other diseases. So is the emergency contraceptive pill, the contraceptive effect can be as high as 98%. But clinically 2% of women who take emergency contraceptive pills still lead to pregnancy, mainly because of individual differences, the amount of drugs absorbed in the body is not the same, so that can not prevent ovarian ovulation, or did not lead to abnormal thickening of the lining of the uterus, did not prevent the fertilization of the egg implantation, which led to pregnancy. The vast majority of patients who take emergency contraceptive pills orally will achieve the desired contraceptive effect, and some will lead to pregnancy.