A 4D ultrasound can be performed at 17 weeks of pregnancy, but the results are less accurate and it is usually recommended that a 4D examination be performed at 20-28 weeks of pregnancy to rule out fetal malformations during pregnancy. Four-dimensional ultrasound is an advanced ultrasound technology that can screen for possible defective diseases such as cleft lip and palate, hydrocephalus, microcephaly and other defective diseases of the fetal face and cranio-cerebral tissues at an early stage, and also assist in ruling out limb malformations, spina bifida, as well as congenital heart defects, hydrocephalus and other pathologies that jeopardize the health of the fetus. Since the amniotic fluid at 17 weeks is not particularly sufficient and the development of the fetus is not yet perfect, the 4-D examination at this time cannot achieve good results and may result in missed diagnosis. After 20 weeks of pregnancy, the rapid development of the fetal brain, fetal structure has been formed, and the amniotic fluid is relatively sufficient, the fetus in the uterine cavity is relatively large space for activities, easy to obtain a clear image, which can more accurately observe the fetal development, so pregnant women should accurately grasp the time of the four-dimensional ultrasound examination. In addition, Down’s syndrome screening can be performed at 17 weeks of pregnancy to determine the risk factor of the fetus suffering from Down’s syndrome and neural tube defects, which is also of great significance in monitoring the development of the fetus during pregnancy.