With the deepening of the application of veterinary B-ultrasound in the veterinary field, scientists also hope to use veterinary B-ultrasound 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. Instead, veterinary B-ultrasound should be used to track and monitor ewes regularly, which can not only monitor the development of the fetus, but also timely detect dead fetuses 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 feeding cost, improve feed utilization, and facilitate production management. In the late 1980s, animal husbandry workers conducted a lot of research on the application of veterinary B-ultrasound to determine the gestational age of ewes. In 1988, Haibei used veterinary B-ultrasound to measure the bifrontal diameter (BPD) of dairy goat fetuses and obtained a formula for estimating gestational age from BPD at 40 to 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 to 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 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 fetal anatomical indicators could more accurately predict gestational age than fetal heart rate. In 1997, Doiz et al. studied the relationship between the caruncle diameter and gestational age of goats and sheep from 30 to 90 days of gestation, 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 fetal anatomical indicators are the best conditions for judging gestational age.

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