The best time for 4D ultrasound is between the 20th and 24th week of pregnancy, that is, between the fifth and sixth month of pregnancy, which is the best time for 4D ultrasound. This is the best time to perform 4D ultrasound because the anatomy of the fetus in this period has been perfected, and 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 skeletal tissues of the fetus in this period have less influence on the exploration of 4D ultrasound, so it is possible to have a more comprehensive understanding of the fetus than in other pregnancy periods. In addition, many fetal anomalies and abnormalities can be revealed during this period. The high resolution of 4D ultrasound can clearly show the anatomy of the fetal tissues and organs and the presence of malformations. 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. Not only is it necessary to have a 4D ultrasound examination, but it is also necessary to go to a hospital with prenatal diagnosis qualification for prenatal diagnosis. In addition, when 4D ultrasound is done between 20 and 24 weeks of pregnancy, not only is the fetus right in all aspects, but also, the pregnant woman is in a more suitable and safe period. Moreover, if the 4D ultrasound detects certain abnormalities in the fetus or the presence of certain malformations, it is within the policy and will not be too troublesome.