In order to prevent the fertilized egg from entering the uterine cavity, oral emergency contraceptive pills can be taken within 72 hours after intercourse to prevent the fertilized egg from entering the uterine cavity, because the fertilized egg formed after the union of the sperm and the egg has to enter the uterine cavity under the peristaltic movement of the fallopian tube to deposit, grow and develop which is pregnancy. It is impossible for a fertilized egg to implant in the abnormally thickened endometrium, so it is prevented from entering the uterine cavity to implant, grow and develop, thus achieving the corresponding contraceptive purpose. However, although the emergency contraceptive pill is effective, it does not prevent 100% of the fertilized eggs from being placed in the uterine cavity, so even if the pill is taken within 72 hours, the fertilized eggs may be placed in the uterine cavity.