4D ultrasound is real-time 3D, and the examination time is 20-26 weeks. At this time, the development of fetal brain tissue, internal organs, limbs and external appearance of the five senses has been basically completed. To do 4D ultrasound, first of all, the fetal development should be routinely observed, and the size of the fetus should be measured, and then the focus should be on ranking the fetus for malformations, such as congenital heart disease, visceral ectopia, duodenal atresia, limb absence or shortening, hydrocephalus, spina bifida, etc. 4D ultrasound can reconstruct the fetus’ face, cranium, spine and heart in three dimensions through 3D reconstruction technology, so that the fetus can be observed more carefully. The amount of amniotic fluid is also relatively large at 20-26 weeks of pregnancy, which can serve as a good window for sound transmission, so that the fetal images can be displayed more clearly, which is conducive to the observation of 4D ultrasound.