There are three reasons why it is not recommended to do 4D ultrasound: 1. It is possible that the time of examination is not suitable, and the normal time for 4D ultrasound examination is 22-28 weeks of pregnancy. The normal time for 4D ultrasound examination is 22-28 weeks of pregnancy. The size of the developing fetus and the amount of amniotic fluid in this period are relatively moderate, which is more conducive to 4D ultrasound examination. If the fetus is too small or too large, it may affect the clarity of 4D ultrasound, so other times are not suitable for 4D ultrasound. 2. It is possible that the doctor suspects that a certain organ of the fetus is developing abnormally, and doing 4D ultrasound may not be as good as simple 3D ultrasound to determine whether there are abnormalities. For example, if the fetus has abnormal heart development, doing 3D ultrasound of the fetus’ heart is more beneficial to the diagnosis. 3. It is possible that the pregnant woman has already done 4D ultrasound examination for many times, and the doctor feels that it is unnecessary to do 4D ultrasound again, and 2D ultrasound as an ordinary pregnancy examination can fully satisfy the needs of the examination.