A 4D ultrasound at 22 weeks can be seen. The time for 4D ultrasound is at 20-28 weeks. The best time for examination is at 22-26 weeks, when the local structure of the fetus has developed well, and 4D ultrasound can detect any serious fetal malformations, such as anencephaly, severe brain bulge, open spina bifida, or pleural wall defect combined with visceral ectopia, single-chambered heart, and lethal chondrodysplasia, and so on. After more than 28 weeks the fetus is more developed and some of them are not easy to see because of thicker fat and tissue obscuring. Therefore, it is recommended to do the examination at 22-26 weeks, because the fetus is of moderate size and the amniotic fluid volume is moderate, and the examination is clearer.