A 3-D exam for pregnant women is not the same as a 4-D exam. In layman’s terms, 3D is equivalent to taking a picture and 4D is equivalent to a small video. The principle of 3D examination is to use Doppler ultrasound, which is equivalent to taking a picture and is static. The 3D can see the fetal face and facial expressions or structural changes, as well as the up and down, left and right, front and back of the fetus in the uterus. It is often used to exclude whether the fetus has developmental abnormalities, such as cleft lip, cleft palate, widened eye spacing, short limbs and toes, how many fingers, abnormal size and shape of both ears, and whether the heart is well developed. The principle of 4D examination is developed on the basis of 3D, which can observe the real-time dynamic image process of fetal activities in the womb, and can clearly see whether there are obvious congenital malformations in the fetal head, spine, limbs and face, and also see whether there are malformations in the fetal brain, kidney and heart, and better observe and find lesions. 3D and 4D ultrasound are not mandatory during pregnancy, 3D and 4D are only dimensions of ultrasound. In fact, ultrasound during pregnancy is divided into 3 levels, and level 3 ultrasound is the key test to perform major abnormalities.