The best clinical time for 4D ultrasound is usually between 20 and 24 weeks of gestation, i.e. between the fifth and sixth month of pregnancy. The fetus will gradually increase in size as the pregnancy progresses, and all tissues, organs and structures will become more and more perfect. During the period of 20 to 24 weeks of pregnancy, the anatomy of the fetus’ tissues and organs has been perfected and can be displayed by the ultrasound; moreover, the space in the uterine cavity, the volume of amniotic fluid and the thickness of the placenta are also suitable for 4D ultrasound examination. Moreover, the fetal skeletal tissues during this period have relatively less influence on the 4D ultrasound investigation, and the images are relatively clear. In addition, many abnormalities and abnormalities of the fetus can be shown during this period. Therefore, it is most appropriate to choose this period for 4D ultrasound examination. The National Health Council also requires screening for anencephaly, severe brain expansion, single-chambered heart, severe open spina bifida, lethal bone dysplasia, and severe thoracic and abdominal wall defects with internal exostosis during this period. In addition, it is necessary to perform 4D ultrasound for those who have suspected fetal malformations during previous routine ultrasound examinations, or for those whose fetuses have high risk factors for malformations. It is also necessary to go to a hospital with prenatal diagnosis qualification for prenatal diagnosis. However, due to the influence of factors such as thick subcutaneous fat in the mother’s abdomen, low amniotic fluid or fetal position, the ultrasound cannot display certain parts of the tissue structure well, so the detection rate of 4D ultrasound for these abnormalities is not 100%. At the same time, ultrasound cannot detect fetal intelligence, hearing, vision and various tissue functions, as well as chromosomal-related diseases. Even fetuses that do not show significant abnormalities on 4D ultrasound may have abnormalities in late pregnancy.