Most of the blood vessels in the small-tailed Han sheep maintain a high degree of stability once they are fully developed. The changes caused by aging are slow and almost imperceptible. Only in the case of disease or trauma can the inherent regeneration ability of blood vessels be aroused. However, the blood vessels of the endometrium are different from the general ones. They replace stability with a high degree of variability, which is more obvious on veterinary B-ultrasound. There are two types of this variability: one is the increase and decrease of blood vessels that occur with the regularly repeated estrus cycle; the other is the irregular increase and decrease of blood vessels due to pregnancy. The adaptability of blood vessels to this necessary procedure determines the different results of normal physiology and clinical pathology: and the factors that produce vascular variability also control the cyclical changes of endometrial parenchyma tissue through blood vessels. Therefore, blood vessels should be regarded as a factor affecting endometrial activity, and blood vessels play a leading role in uterine physiology. It is also important to observe the changes of blood vessels through veterinary B-ultrasound.

One or several branches of the uterine artery and ovarian artery penetrate the outer edge of the uterine wall at an oblique angle and reach the middle layer of the uterine myometrium. The arched arterial crown is formed at this branch. Radial arteries emanate from the crown of the arcuate artery, which penetrate the endometrium in a perpendicular course to the uterine cavity. Just after passing through the junction of the myometrium and the endometrium, each radial artery is divided into the basilar artery and the endometrial spiral artery, which is also more obvious when observed on veterinary B-ultrasound. The basilar artery supplies the basal layer of the endometrium, which is not affected by hormonal stimulation; the spiral artery supplies the functional layer of the endometrium and is extremely sensitive to the influence of hormones. The endometrial spiral arteries bear most of the functional activities of the endometrium. During pregnancy, the uterine blood vessels become thicker and more branched, especially the middle uterine artery and the uterine branch of the vaginal artery, which is also obvious when observed on veterinary B-ultrasound. As the blood vessels become thicker, the wrinkles of the arterial intima increase and thicken, and the connection with the muscular layer is loose, so when the blood flows, it changes from the original clear pulsation to intermittent and inconspicuous tremor, which is called the pregnancy pulse. After pregnancy in all animals, the size and weight of the uterus increase. In the first half of pregnancy, the increase in uterine volume and weight is mainly caused by the hyperplasia of uterine muscle fibers: in the second half of pregnancy of small-tailed Han sheep, it was mainly due to fetal growth and increased amniotic fluid.

After the sheep is pregnant, the blood supply to the uterine mucosa increases due to the effect of progesterone, the mucosa thickens, and the connection between the mucosa and the submucosal tissue becomes loose. Therefore, a large number of wrinkles can form in the uterine mucosa in the early stage of pregnancy. When the uterus gradually increases, the wrinkles flatten and disappear. This phenomenon is more obvious when observed on veterinary B-ultrasound. The uterine glands also expand, stretch, and curl, and the glycogen in the glandular cells increases and the secretion increases. This secretion is the main component of uterine milk. After the fetal sac is attached, the sheep uterine mucosa forms a cotyledon-type placenta, and the cotyledons are more obvious on veterinary B-ultrasound. The uterine caruncle develops into the maternal placenta. The pregnant horn placenta is larger than the non-pregnant horn, and the placenta at the base and middle of the pregnant horn is larger than the rest. At the end of pregnancy, the uterus occupies the right half of the abdominal cavity and part of the left side, and the rumen is squeezed to the left front; at this time, due to the expansion of the uterus, the thinning of the uterine wall is most obvious, and it can be measured by veterinary B-ultrasound to reach 1mm.

After the sheep is pregnant, veterinary B-ultrasound can observe that the cervix of the ewe is tightened and the mucosa is thickened. The single-cell glands of its epithelium secrete sticky mucus under the influence of progesterone, which fills the cervical cavity, called cervical plug. Therefore, the cervical canal is tightly closed to prevent bacteria from entering the uterus and protect the safety of the fetus. The mucus is transparent and pale at first, and then turns grayish yellow, becomes more viscous, and the amount increases, and enters the vagina.