During the whole pregnancy, you usually need to do 2 major 4D and 1 minor 4D between 11-13 weeks. The minor 4D refers to NT examination, if the thickness of NT exceeds the normal range, it indicates the possibility of fetal developmental abnormalities and further prenatal diagnosis is needed. 4D ultrasound is the most important test for understanding fetal growth and development and screening for fetal abnormalities during pregnancy. 4D ultrasound can provide a more systematic and comprehensive understanding of the development of each organ of the baby. If some abnormal indicators are found in the middle of this examination process, further examination and treatment should be carried out during the period according to the situation.