When the baby is in the supine position or breech position, the baby’s spine may not be visible during the 4D. 1. Supine position: If the baby is in the supine position, that is, the back of the fetus is close to the mother’s back, it may be beyond the range of the ultrasound probe. 2. Breech position: If the baby is in a breech position, the lower end of the fetal spine is close to the pelvic inlet, which may also cause the fetal spine to be invisible during the ultrasound examination. If the spine cannot be seen clearly during the examination due to the above reasons, it is recommended that the pregnant woman performs appropriate physical activities such as climbing stairs, etc., after which the position of the fetus may be changed so that the spine can be seen clearly. If the spine is still not visible, the examination can be repeated at a later date.