The best time to do 4D ultrasound is between 24 and 28 weeks of pregnancy. Before six months of pregnancy, the fetus is relatively small and not mature in many aspects, so it is not possible to do ultrasound to see whether the fetus is developing normally and whether there are malformations. After the 28th week of pregnancy, the fetus is larger and has less space to move around in the uterine cavity. Sometimes the face and limbs of the fetus will be affected, so it will not be clearly observed during the 4D ultrasound, which will also affect the judgment. During the 24-28 weeks of pregnancy, the size of the fetus is moderate and the amount of amniotic fluid is relatively large, which can be used as a good sound transmission window to observe the fetus. The structure of the fetus’s trunk, limbs and head has basically formed at this time. Having 4D ultrasound can assist doctors in diagnosing whether there are malformations in the fetus’s development, including facial malformations, internal organ malformations and limb malformations. The 4D ultrasound can also dynamically observe the fetus’ activities in the womb, and the pregnant mother can also observe many of the fetus’ movements through the screen, such as yawning, sticking out the tongue, swallowing, etc., which is very visual.