Level 3 screening is not just about 4D, level 3 ultrasound screening is systematic prenatal ultrasound screening. The indications for level 3 screening are all fetuses at 20-24 weeks of pregnancy, NT thickening in early pregnancy, estimation of gestational weeks to assess fetal growth and development, screening and follow-up of fetal anomalies, multiple pregnancies, follow-up of placenta previa, placental abruption, suspicion of amniotic fluid abnormalities, cervical insufficiency, pre-cervical cerclage, pre-amniocentesis, uterine malformations and so on. In addition to the routine items of weekday maternity checkups, the examination also includes fetal head, face, spine vertebrae, heart, abdominal organs, umbilical cord, and placenta, which can screen most of the malformations of the fetus. The most suitable pregnancy week for 4D examination is between 22 and 28 weeks, which can observe the fetus dynamically and three-dimensionally, and is able to measure the size of the fetus and determine whether it is consistent with the pregnancy week. 4D examination is effective in detecting the umbilical cord blood flow, umbilical cord, placenta and amniotic fluid. Level 3 screening includes 4D examination, but is not limited to 4D ultrasound. Both three-level screening and four-dimensional examination are important examinations during pregnancy, which can detect various conditions of the fetus. Pregnant women should complete the maternity examination on time during pregnancy to keep abreast of the development of the fetus.