After pregnancy, the uterus of all animals increases in size and weight. In the first half of pregnancy, the increase in uterine size 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 pregnancy, the blood supply of the uterine mucosa increases due to the action of progesterone, the mucosa thickens, and the connection between the mucosa and submucosal tissue becomes loose. Therefore, a large number of wrinkles can form in the uterine mucosa in the early stage of pregnancy. As the uterus gradually increases, the wrinkles flatten and disappear. This phenomenon is more obvious when observed in sheep 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 sheep 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 B-ultrasound in sheep up to 1mm.

After pregnancy, B-ultrasound can be used to 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.

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