The placenta is generally required to be more than 7cm from the cervical os, and 5cm is a relatively safe range. Nowadays, placenta is no longer reported (confirmed) as long as the placenta is more than 2cm from the cervical os on ultrasound. The main risk of placenta praevia is the possibility of painless vaginal bleeding by late pregnancy, where the fetal baby bleeds from the abruption of the placenta before delivery. Therefore, if the placenta is close to the internal cervical os, or if it covers and obscures the internal cervical os, it is called placenta praevia. A fetal baby with an anterior placenta may not reach full term and may be born prematurely. In contrast, the position of the placenta is relatively safe, and by the time of delivery, there is less chance of postpartum bleeding. Because the placenta is attached to the inner wall of the uterus, the body of the uterus and the myometrium of the uterus are more numerous, the uterus contracts better after delivery and the hemostasis is more effective. The cervical opening has more fibrous connective tissue and less muscle tissue, so the chance of cervical contraction after delivery is a little worse.