With the deepening application of Veterinary Ultrasound machines in the veterinary field, scientists also hope to use veterinary Ultrasound Machines to diagnose pregnancy and determine the number of fetuses, as in human medicine, and also to identify the gestational age of the fetus. Because in actual situations, in order to reduce the cost of feeding, breeders cannot put a large group of ewes in estrus at the same time and then mix them with rams for breeding records. Instead, they simply mix the estrus ewes with rams and breed them randomly, so it is impossible to judge the pregnancy of the ewes. The use of veterinary B-ultrasound machines to conduct regular tracking and monitoring of ewes can not only monitor the development of the fetus, but also timely detect stillbirth or fetal development abnormalities, and determine the gestational age of the fetus by measuring various fetal indicators. In this way, ewes with different pregnancy levels and numbers of fetuses can be raised separately, which will greatly reduce the cost of feeding, improve feed utilization, and facilitate production management.

In the late 1980s, animal husbandry workers conducted a lot of research on the use of veterinary B-ultrasound machines to determine the gestational age of ewes. In 1988, Haibei used Veterinary B-ultrasound machine to measure the bifrontal diameter (BPD) of dairy goat fetuses and obtained a formula for estimating gestational age from BPD at 40-100 days of gestation, and believed that breed, fetal sex and number of fetuses did not affect the accuracy of the formula. In 1988, Kelly and Newnham used the occipital-mouth length of the fetal head and BPD to determine the gestational age of Merino sheep, and believed that 40-80 days of gestation was the best time to determine the gestational age. The 95% confidence interval for determining gestational age by fetal head length was ±2 days at 40 days of gestation and ±4 days at 80 days of gestation. In addition, during this period, the prediction curve of fetal head length was applicable to both single and twin fetuses, and was not affected by fetal sex and nutritional status of the ewe, but there may be differences in breed, sex, equipment and operators. In 1990, Sergeev et al. divided ewes pregnant with single, twin and triplets into groups to study the relationship between fetal head width and chest depth and gestational age. They obtained linear and quadratic regression equations for head width and chest depth in each group from 49 to 119 days of gestation and believed that they could be used to determine gestational age. In 1992, Aiumlamai et al. monitored the development of internal organs of 7 Swedish short-wool sheep fetuses from 1.5 to 2 months of gestation to delivery and studied their correlation with gestational age by measuring heart rate, BPD and body width. The results showed that although data were not easy to obtain in some cases, BPD and body width were highly correlated with gestational age, while heart rate was not highly correlated with gestational age because the heart rate of the fetus tended to be stable in the late pregnancy. In 1996, Moreno et al. used more indicators: fetal heart rate, body length, head width, eye spacing, lateral height from the thorax to the diaphragm and the caudal area of the sternum, and the diameter of the thoracic and abdominal cavities to study their relationship with gestational age. They believed that the number of fetuses had an important influence on the establishment of the regression equation, and that the fetal anatomical indicators could more accurately predict the gestational age than the fetal heart rate. In 1997, Doiz et al. studied the relationship between the caruncle diameter and gestational age of goats and sheep during gestation

30 to 90 days, and obtained the relationship between gestational age and caruncle diameter. Then they verified the accuracy of the relationship. The results showed that the accuracy was 66% within the range of ±7 days and 96% within the range of ±14 days, while the caruncle diameter of sheep had little correlation with gestational age. It can be seen that the anatomical indicators of the fetus are the best conditions for judging gestational age. Comprehensively, the application of veterinary B-ultrasound machine in the three aspects of sheep pregnancy monitoring shows that with the continuous improvement of veterinary B-Ultrasound Equipment and the in-depth development of ultrasonic diagnostic technology, veterinary B-ultrasound machine will be more widely used in livestock breeding and obtain more satisfactory results.

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