The difference between 2D ultrasound and 3D or 4D ultrasound is mainly the difference in imaging. Two-dimensional ultrasound imaging is a flat image, while three-dimensional or four-dimensional ultrasound imaging is a three-dimensional image. Three-dimensional imaging is a three-dimensional visual image obtained by computer processing of many two-dimensional line and surface images. Four-dimensional ultrasound is real-time three-dimensional ultrasound, that is, dynamic three-dimensional ultrasound. At present, high-end ultrasound devices have 3D or 4D ultrasound functions, which are usually used in screening for fetal malformations in mid to late stage pregnancies to obtain intuitive images of the fetus, as well as to obtain more planes that cannot be obtained by ordinary 2D ultrasound to help doctors diagnose fetal malformations. For example, the diagnosis of malformations of the skull and brain, and the diagnosis of malformations such as corpus callosum resection. Pregnant women in the hospital for ultrasound examinations are basically 2D ultrasound, which is simple to operate and is used to check the development of the fetus and the exclusion of malformations. 3D and 4D ultrasound can be optional, but 2D ultrasound must be checked regularly, and fetuses found to have problems in 2D examination must be further confirmed by 4D ultrasound examination.