4D ultrasound can be done at 5.5 months. 5.5 months, i.e. 24 weeks of pregnancy, is the best time for 4D ultrasound examination. Generally speaking, the best time for 4D ultrasound is between 20-28 weeks of pregnancy, because after 18 weeks of pregnancy, the fetus’s organs and organs have developed, the amniotic fluid volume is sufficient, and the fetus’s mobility inside the uterus is larger, so that the fetus’s various indicators of growth and development can be observed comprehensively and clearly, and the screening of fetal malformations can be carried out. If the pregnancy week is too big or too small, it may affect the accuracy of 4D ultrasound examination. Four-dimensional ultrasound can surface imaging, for fetal cleft lip and palate, limb developmental deformities, meningoencephalocele, spina bifida, abdominal wall bifida and other congenital malformations can be clearly checked out, but also for congenital heart disease can be done early diagnosis, is a big screening test in pregnancy.