Most of the structures of the fetus in the fourth month are already full-grown, and ultrasound can observe a lot of contents, as follows: 1) observation of the number of fetuses, such as the number of fetuses; 2) observation of the fetal orientation, that is, the fetal head orientation can be observed; 3) observation of the size of the fetus, by measuring the biparietal diameter, head circumference, abdominal circumference and femur of the fetus, estimating the gestational week and the weight of the fetus; 4) The approximate anatomical structure of the fetus can be observed, such as the condition of the head, the condition of the spine, the continuity of the abdominal wall, the soundness of the long bones of the limbs, and also the organs in the abdominal cavity of the fetus, such as the gastric vesicles, the liver, and the bladder; 5. Some of the appendages of the fetus can be observed, such as the position of the placenta, the maturity of the placenta, the position of the placenta from the inner cervical opening, and the observation of the amniotic fluid; 6. The normal fetal heart rate is 120-160 beats per minute. There are many structures that can be observed by ultrasound in the fourth month of fetal life.