The veterinary B-ultrasound echo state of the placental process of small-tailed Han sheep changed after 14 weeks of pregnancy, which may be related to the continuous branching and development of fetal villi and further connection with the mother. Fetal villi are rich in blood vessels. As they continue to branch and erode the maternal placenta during development, the maternal placenta part that was originally highly echogenic in the placental process gradually darkens in the veterinary B-ultrasound images, which is the result of the gradual development of the placenta. It has been reported that the diameter and weight of the placental process of sheep with single fetuses will gradually decrease in the middle and late stages of pregnancy. Some people believe that this change in the placental process may be related to the increase in placental perfusion and tissue permeability in the middle and late stages of pregnancy.

Through veterinary B-ultrasound observation, there is a statistical relationship between the diameter of the placental process and the number of weeks of pregnancy in the first 11 weeks of pregnancy. However, the statistical results obtained by different authors are not the same, which can be explained by the different sheep breeds and the settings of independent and dependent variables. In the first 11 weeks of pregnancy, there is a linear relationship between the diameter of the placental process and the number of weeks of pregnancy. It can be seen that the diameter of the placental process in the first 11 weeks of pregnancy can also be used to identify the gestational age. It should be noted that the diameter of the placental process varies greatly in different parts of the fetal membrane. Generally, the diameter at the top of the uterine horn is smaller, the diameter at the intersection of the two cervixes is larger, and the diameter at the pregnant horn is larger than that at the empty horn. Therefore, choosing the measurement site and measurement method is crucial to reduce errors and increase accuracy. Selecting the placental process around the fetus for measurement through veterinary B-ultrasound can, on the one hand, reduce the difference between ewes with single and multiple pregnancies. On the other hand, because the diameter of the placental process here is relatively large in order to provide nutrition for fetal development, the error of the result obtained through multiple measurements each time is relatively small, and the accuracy will be slightly higher.