The lower edge of the placenta reaches the endocervical os at 12 weeks, indicating that the position of the placenta is low, which is the state of placenta previa, and needs to be observed dynamically in accordance with medical advice. Ultrasonography at 12 weeks of pregnancy suggests that the lower edge of the placenta reaches the endocervical os, indicating that the position of the placenta is low, known as placenta previa, which may be caused by high-risk factors, such as previous uterine operations, multiple pregnancies and deliveries, and a history of puerperal infections. With the increase of gestational weeks and the formation of the lower uterine segment, most of the placentas suggestive of low position in the middle trimester change to normal position with the upward movement of the uterine body, but a small number of pregnant women develop low placenta or placenta praevia. Patients need to pay attention to appropriate rest, avoid exertion and strenuous exercise, prohibit coitus, so as not to increase the risk of bleeding, but also comply with the doctor’s instructions for dynamic observation, and review on time.