Emergency contraceptive pills may also delay menstruation, the principle of action of emergency contraceptive pills is to inhibit ovulation, resulting in changes in the endometrial lining of the uterus, the increase of cervical mucus plugs, and at the same time can cause endocrine disorders. The principle of emergency contraception is to avoid ovarian ovulation, can play the role of emergency contraception, so in the inhibition of ovarian ovulation at the same time may be delayed menstruation. Emergency contraceptive pills delayed menstruation phenomenon, but also does not rule out the emergency contraceptive pills failed to cause the state of pregnancy. The success rate of emergency contraceptive pills is only about 60%, and there is a 40% chance of causing an unwanted pregnancy. Therefore, if there is a delay in menstruation after taking emergency contraceptive pills, it is best to go to the hospital for examination to rule out the possibility of pregnancy.