The 4-D ultrasound can observe the fetal growth and development in the uterus from multiple directions and angles, not only to feel the fetal breathing and movement, but also to let pregnant mothers see the fetal series of movements more clearly and intuitively, so that parents who do not know the ultrasound images can also see the fetal appearance. 5 months old fetus and amniotic fluid are moderate in size, and there is more space for intrauterine activities, and the fetal bone echo image is relatively small, so the 4-D ultrasound image is relatively clear. The fetus is small, about 19-22cm in height and 350-400g in weight, and the fetus does not have a lot of subcutaneous fat and is relatively thin. When the position is good, the fetus’ eyes, nose, mouth and ears are clearly discernible; the shape and number of fingers and toes can be displayed; there are more small movements of hands and legs, such as scratching the nose, rubbing the face, patting the face, sometimes pouting and chest heaving, which is the fetus breathing, but what is inhaled and exhaled is not air, but amniotic fluid. Although 4D ultrasound can see the sex of the fetus at this time, the state regulations prohibit fetal sex identification for non-medical purposes, so it is illegal to conduct fetal sex identification for non-medical purposes in China.