Lower margin covering the endocervical opening at 12 weeks generally means that the placenta is positioned low enough to reach or cover the endocervical opening. 12 weeks belongs to the early pregnancy, at this time the placenta position is low, do not need to cause excessive concern, it is recommended to regularly review the ultrasound. Generally with the increase of pregnancy weeks, the uterus gradually increases in size, the fetus continues to grow, the position of the placenta will be relatively upward, the distance between its edge and the endocervical os will gradually increase, generally to the middle and late pregnancy, i.e., about 28 weeks of pregnancy, the placenta’s position will be relatively fixed. Therefore, if the ultrasound indicates that the placenta is in a low position at 12 weeks of pregnancy, it is recommended that the ultrasound be repeated periodically. If the ultrasound still indicates that the lower edge of the placenta is covering the cervical opening after 28 weeks of pregnancy, placenta praevia should be considered, and attention should be paid to it, and timely medical treatment should be sought.