4D usually refers to four-dimensional ultrasound during pregnancy, and it is usually better to choose to have the test around 24 to 28 weeks of pregnancy. Four-dimensional ultrasound belongs to a type of ultrasound examination, also known as a detector test. In addition to normal ultrasound functions, it can also carry out three-dimensional imaging of the head and face of the fetus, so as to show the state of the fetus’s eyes, nose, mouth, jaw and so on, which can assist the doctor to diagnose the congenital anomalies of the fetus. Pregnant women should understand the importance of 4D ultrasound screening and go to the hospital for 4D ultrasound according to the time prescribed by the doctor. 4D ultrasound can be performed at 24~28 weeks of pregnancy because at this time, the fetal structure has been constituted, and the size of the fetus and amniotic fluid are moderate, the image is clearer, which is conducive to diagnosis. It is recommended that pregnant women should go to the hospital at the prescribed time after pregnancy to have a 4D ultrasound examination, so as to understand the development of the fetus in the womb.