4D ultrasound is done between 5 and 6 months of pregnancy, that is, between 21 and 25 weeks of pregnancy, preferably between 22 and 24 weeks of pregnancy. At this time, all organs of the fetus have been developed, its body length has reached 30cm, top and hip diameter has reached 21cm, and its weight has reached 630g, and there is more amniotic fluid at this time, so the fetus has more space to move freely in the uterus. At this time, some of the fetal organs are not fully mature, and the 2D ultrasound can be performed again at 32 weeks of pregnancy to systematically check the fetal structure for malformations.