If the lower edge of the placenta is more than 2cm from the endocervical opening, it is considered to be within the normal range. If it is less than 2 cm, it is called a placenta praevia. If the lower edge of the placenta is close to or covers the opening of the cervix, it is called placenta previa. However, because the uterus will keep enlarging during pregnancy, the lower part of the uterus will also keep elongating. If the ultrasound examination in mid-pregnancy indicates placenta praevia or placenta previa, there is still a chance that the placenta will move up to the normal position in the late pregnancy, i.e., after 28 weeks, which should be determined by ultrasound examination after 28 weeks of pregnancy. The final determination of the position of the placenta is based on the results of the last examination before labor to determine its classification.