The 4D ultrasound is performed between 20-28 weeks, with the best time generally being between 22-26 weeks. Depending on the results of the patient’s first NT, the patient can discuss with the doctor a second exam at 22-26 weeks. The 4D ultrasound needs to be done three times throughout the pregnancy, the first time at 11-13 weeks of pregnancy, a mini-4D is done to look at the NT thickness. The second time, at 22-26 weeks of pregnancy, the first major 4D is done. The third time around 30-32 weeks of pregnancy, do the third big four-dimensional. 22-28 weeks between the fetal structure has been fully formed, and the size of the fetus and amniotic fluid are more appropriate, and in the uterus activity space is larger, the impact of fetal bone echo is smaller, the image is relatively clear, can focus on the detection of the development of various fetal organs malformations. Therefore, the second four-dimensional ultrasound general examination is recommended to choose the best 22-26 weeks to carry out, it is not recommended to exceed 26 weeks. In short, four-dimensional ultrasound is a major examination, can check the fetal body surface, but four-dimensional ultrasound examination is more nervous, need to make an appointment in advance, pregnant women need to pay attention to avoid missing the best time.