Under normal circumstances, ultrasound can be done once every 2-4 weeks after 28 weeks of pregnancy and once every 1 week after 37 weeks of pregnancy. Generally, after the 28th week of pregnancy, it is called late pregnancy. Ultrasound is a non-invasive examination of the growth and development of the fetus, which can find out the size, position, umbilical cord blood flow, amniotic fluid and other conditions of the fetus. Under normal circumstances, if the fetus is growing and developing well, and the pregnant woman does not have any abdominal pain, tightness and hardness of the abdomen, vaginal bleeding and running water and other uncomfortable symptoms, ultrasound can be performed once every 2-4 weeks, and after 37 weeks of pregnancy, ultrasound can be performed once a week. Around the expected date of labor, at this time, we need to be alert to the umbilical cord blood circulation is insufficient to cause the fetus to appear intrauterine hypoxia, may need to carry out frequent ultrasound examination. The exact timing should be considered in the context of the individual situation of the pregnant woman and varies from person to person. In the late stage of pregnancy, you should pay attention to follow the doctor’s instructions for regular checkups, and if there is any abnormality or discomfort in the middle of the pregnancy, ultrasound examination can be performed at any time.