The term four-dimensional is a spatial concept, from the space of the point, surface, body of the three-dimensional plus time dimension constitutes the concept of four-dimensional; four-dimensional ultrasound refers to the stereoscopic image ultrasound based on the dynamic three-dimensional image can be displayed, which is the concept of four-dimensional ultrasound. Ultrasound is to probe the body containing tissue structures within the skin, and is cut, just like cutting a watermelon, by cutting not only to see the seeds inside but also to see the kernel inside the seeds. 3D ultrasound is a three-dimensional image of an object displayed by scanning information in three consecutive sections, while the surface 3D image of an object in the body is viewed through the human body, which can not be displayed by the method of exploration of sections only. 4D ultrasound is most suitable for fetal examination, and the images of other diseases are slightly worse. For example, the 4D ultrasound view of the three-dimensional surface image of the fetus in the pregnant woman’s uterus is a combination of opaque display and volumetric data to show the surface anatomy of the fetus. Not only is the fetus detected in the body through the cut surface ultrasound, but also the three-dimensional surface of the fetus is displayed without showing the cut surface echoes in the fetus, together with the dynamic image, which is what I understand by 4-dimensional ultrasound. So it is understandable that 4D ultrasound can detect lesions and defects in the fetal form such as cleft lip and palate, spina bifida, abdominal wall defects, etc. This technology is very advanced and has different display modes such as fetal skeletal anatomy display mode, etc.