The lower edge of the placenta reaching the cervical opening at 21 weeks of pregnancy may be due to the relatively large placenta, delayed development of the trophoblast of the fertilized egg, and endometrial lesions. 1. The placenta is relatively large: at 21 weeks’ gestation, the placenta is relatively large in relation to the uterine cavity, and there may be a phenomenon of the placenta’s position being relatively low, with the lower edge reaching or covering the cervical os. With the increase of pregnancy months, the placenta position may gradually rise up to reach the normal position. 2. Delayed development of the trophoblast of the fertilized egg: when the trophoblast is not developed to a stage where it can be placed on the uterus, the fertilized egg will continue to move down to the lower uterine segment, which may result in the lower edge of the placenta reaching the opening of the cervix. 3. endometrial lesions: some women have multiple abortions, repeated purging, may lead to endometrial lesions, resulting in the fertilized egg absorption of nutrients is insufficient, so that the fertilized egg in the uterus in the lower part of the bed, with the month of pregnancy, the lower edge of the placenta is also extended to reach the cervical position. The lower edge of the placenta reaches the cervical opening at 21 weeks of pregnancy, indicating that the placenta is in the state of placenta previa, it is recommended to pay more attention to rest, avoid strenuous exercise, pay more attention to observe whether there is vaginal bleeding phenomenon, and regular obstetric examination.