To do 4D ultrasound to choose the right time, the best time for 4D ultrasound is 22-28 weeks (try to be within 26 weeks), this period of time for examination, because the fetus is smaller, amniotic fluid is moderate, the fetus has a large degree of mobility, and the surface of the body is plump, which is conducive to 4D ultrasound imaging of the fetus and fetal anomaly screening, and it is a period for malformation elimination. Therefore, it is not too late to do it at 22 weeks. Some people may ask whether it can’t be done after 28 weeks, in fact, it’s not. Four-dimensional ultrasound is mainly a test for fetal malformation detection, so after 28 weeks, due to the skull, limbs and other bones are better developed, the ultrasound will have a relatively large attenuation effect, which is not conducive to the doctor’s observation of the fetal structure, the doctor can’t make an accurate judgment on the fetus with or without malformations. Therefore, usually after 28 weeks, doctors will not do this test.