1.Early stage of pregnancy: It can show the complete gestational sac and visualize the embryo in its early human form. 2.Mid-term (24-32 weeks): It can clearly display all parts of the fetal organs, understand the growth and development of the fetus, and observe whether there are malformations in the head, limbs and general structure of the organs. Due to the perfect development of the fetus in this period, the amniotic fluid volume increases, three-dimensional imaging in the liquid surrounded by the conditions of better results, can show most of the tissues and organs of the structure and the entire development of the fetus, the period of the fetus face plump, clear senses, is the best period of time for diagnosis, imaging success rate is high, the most meaningful to the clinical diagnosis. 3.Termination of pregnancy: It can accurately determine the fetal position, check the maturity of the placenta, whether the umbilical cord is wrapped around the neck, determine the hemodynamic changes in the umbilical artery index (CDFI, S/D), and evaluate whether the fetus is hypoxic in the uterus. 4. Post-delivery examination: to observe whether the placenta is retained and the uterus is restored. Therefore, pregnant women should carry out the necessary perinatal checkups at these four specific stages under the guidance of doctors to provide a reliable basis for treatment.