Generally speaking, there are four ultrasound examinations during pregnancy. The first one: 11-14 weeks of pregnancy, mainly to see the size of the fetus (individual differences in fetal size are very small between 12-13 weeks of pregnancy, which can be used to check the week of pregnancy and project the expected date of delivery), whether there are any major abnormalities, to measure the thickness of the nuchal translucency, and to predict the risk of fetal chromosomal disorders. The second time: 20-24 weeks of gestation, mainly to see if the fetus has any malformation. This time, the ultrasound not only measures the fetal biparietal diameter, head circumference, abdominal circumference and femur length to estimate the fetal size, but also looks at the fetal brain, upper lip line, four-chambered heart, gastric vesicles, both kidneys, bladder, long bones of the limbs, as well as whether the spine is continuous and complete, the thickness of the subcutaneous fat layer of the neck, whether the skin of the abdominal wall is complete, whether the intestinal canal is dilated, whether the blood vessels of the umbilical cord are The third time: 28-32 weeks of pregnancy. The third time: 28-32 weeks of pregnancy, mainly for further screening of abnormalities, some abnormalities such as cardiac macrovascular abnormalities, dilated intestinal canal and anal atresia can be detected at this stage. To find out whether the fetus is delayed or too large, to check whether there is abnormal fetal position, whether the placenta is anterior, whether the amniotic fluid volume is abnormal, whether the umbilical cord is wrapped around the neck, etc. The S/D value is also often measured. The fourth one: 37-38 weeks of pregnancy, mainly to see if the fetus is mature, in addition to fetal position, amniotic fluid and placenta examination are also included.