Generally, the time to do 4D is between 20 and 24 weeks, which is a more ideal time for fetal teratology screening, because this time can perform whole body testing, such as head, trunk and limbs, and can clarify whether there are cleft lip, cleft palate or neural tube malformations. Another good time is from 28 to 32 weeks, when the second screening can be performed if necessary, such as heart malformations, which can be seen more clearly; ultrasound can also be performed in late pregnancy, but it is not particularly convenient to do 4D ultrasound due to the obscuring relationship, so the time to do 4D ultrasound is from 20 to 24 weeks and 28 to 32 weeks.