The measurement of fetal chest width and abdominal width cannot be measured as recorded in other literatures due to the characteristics of the 3.5MHz sheep B-ultrasound probe (the sound beam emitted is fan-shaped rather than linear, and the Sheep B-ultrasound probe cannot be rotated 90 degrees to obtain an image perpendicular to the original section). They are also good indicators for fetal age identification.

The fetal BPD, chest width and abdominal width measured before delivery are slightly lower than those measured in newborn lambs. The reasons are as follows: the values measured by sheep B-ultrasound do not include the skin; the mutual squeezing between fetuses in the mother's uterus and the compression of the mother's abdominal wall and probe; sucking immediately after birth increases the abdominal circumference.

The consistency of fetal development in early pregnancy in sheep B-ultrasound examinations has been widely confirmed. Although there are differences in fetal development caused by breed, fetal sex and number of gestations, they are not obvious before 80-90 days of pregnancy. The differences in BPD between three breeds of goats and two breeds of sheep were compared. The results showed that the differences in BPD were not obvious at the beginning of the second trimester, but began to show differences at the end of the second trimester. In the second trimester (40-100d), the BPD formula of Suffolk sheep can be used to identify the gestational age of Finn sheep. This indicates that there may be breed differences between Small-tailed Han sheep and other sheep breeds, but further confirmation is needed.

Since the fetal skull is easy to distinguish from the surrounding tissues in sheep ultrasound images, changes constantly throughout the pregnancy, and is closest to the actual measurement value with a small error, the fetal BPD can be used as the most appropriate indicator for gestational age identification.

The head length and BPD of the Small-tailed Han sheep fetus, especially the fetal BPD, are the most commonly used indicators for gestational age identification. At the same time, there are reports of using fetal BPD to identify gestational age in rhesus monkeys, alpacas, dogs, cattle, etc.

veterinary B-ultrasound generally uses a 3.5MHz probe, and the time when the fetal head length and BPD can be measured is generally in the 6th and 7th weeks of pregnancy, respectively. Some people will measure earlier, which may be caused by the different frequencies of veterinary B-ultrasound probes. Some people will use 5.0MHz veterinary B-ultrasound probes. In the early stages of pregnancy, the uterus of small-tailed Han sheep is still at the front edge of the pelvis and its vicinity. At this time, the use of 3.5MHz probes can overcome the problem of low penetration of 5MHz probes, and the state of the entire uterus can be clearly seen under a wider field of view, but due to the low resolution of 3.5MHz sheep B-ultrasound probes, the accuracy of the measured values is not high. From the 9th week of pregnancy, characteristic BPD images of the fetus appear. At this time, different sections need to be taken to measure BPD and head length, and the accuracy of the measured values begins to gradually increase. Due to the characteristics of the 3.5MHz sheep B-ultrasound probe, the entire monitoring period is longer than that of the 5MHz sheep B-ultrasound probe. The fetal head length exceeds the detection range of sheep B-ultrasound in the 13th week, and the fetal BPD can be measured throughout the pregnancy. Therefore, the development law of the fetal skull and cranial cavity throughout the pregnancy is obtained. However, in late pregnancy (after 100 days), due to the characteristics of multiple pregnancies in small-tailed Han sheep, the fetal head can be located anywhere in the abdominal cavity during this period, which brings considerable difficulty to measurement. On average, each measurement takes more than 20 minutes. Therefore, from the perspective of fetal age identification, the best time to use fetal head length and BPD to identify fetal age should be in the middle of pregnancy (50 days-120 days).