4D ultrasound is generally best done during the period of 22-26 weeks of pregnancy. The reason is that during this period, there is more amniotic fluid, the fetus is not very large, and the ratio of the fetus to the uterine cavity is relatively good, so the 4D ultrasound can be performed at this time to see all the structures of the fetus relatively clearly. Nowadays, some hospitals call it 3-D ultrasound, 4-D ultrasound, and some hospitals call it tertiary ultrasound. In fact, if you choose one of these three types of ultrasound, you can do it, they all have the same meaning. If there is a problem with 3D or 4D, a large hospital may need to do the highest level of ultrasound, which is tertiary ultrasound, which can be more accurate. If the ultrasound reveals a problem with the fetus, further prenatal consultation or prenatal diagnosis will be required.