The viscera of the fetus of the small-tailed Han sheep, after the embryonic development enters the fetal stage, the anatomical structures in the body have basically formed. Although they can gradually increase, mature and function as the pregnancy progresses, their anatomical positions have been basically determined as early as the fourth week of pregnancy. Most of the fetal internal organs can be displayed by veterinary B-ultrasound, which mainly depends on the reflectivity of the anatomical structure, the resolution of the veterinary B-ultrasound instrument and the veterinary B-ultrasound exploration technology. The echo-free structures are mostly fluid-containing organs, which appear as liquid dark areas, such as the cardiovascular, bladder, gastrointestinal, etc., belonging to multiple anatomical systems. The medium or low echo structures are mostly solid organs such as the liver, kidneys, and brain. As the fetus develops, their performance in the veterinary B-ultrasound images has also undergone a change process. It takes until the middle and late stages of pregnancy to gradually show the same echo characteristics as adults, and veterinary B-ultrasound can clearly show the edges of various organs. In addition, some anatomical structures of the fetus are different from those of the adult. Mastering these characteristics is also beneficial and necessary for the exploration of the fetal viscera.

The earliest time for detecting the appearance of the organs and tissues of the Small Tail Han Sheep fetus is relatively late, which is due to the frequency of the veterinary B-ultrasound probe. The use of a 3.5MHz veterinary B-ultrasound probe in the middle and late stages of pregnancy increases the scanning range and reduces the resolution. At the same time, because the scanning range of the veterinary B-ultrasound probe is greater than that of the 5.0MHz veterinary B-ultrasound probe, when many organs have exceeded the scanning range of the 5.0MHz veterinary B-ultrasound probe, the veterinary B-ultrasound probe can still play a good role in monitoring the development of various organs in the late stages of pregnancy.