Embryonic development is initiated by the fusion of male and female pronuclei. About 30 hours after fertilization of dairy cows, the zygote undergoes the first cleavage, and on the 3rd day, the blastocyst enters the uterus. In the second week, the conceptus extends and sends a signal of its existence to the mother. Around the 30th day, the embryo attaches to the uterine wall. The embryonic period ends on the 42nd day. After that, the conceptus is called a fetus. At this time, the main tissues, organs and systems of the fetus have been formed. There is also fluid in the uterine horns of non-pregnant dairy cows during estrus, which may be mistaken for blastocyst fluid. Therefore, ultrasound images of the embryonic sac before 20 days of pregnancy are unreliable. Using B-ultrasound machine images to classify dairy cows, the embryos of 20-24 days are short and straight, 25-26 days develop into a C shape, and 32 days become L-shaped. The head and body of the fetus can be distinguished at 33 days. According to comprehensive reports, the first observation time of the physiological characteristics of dairy cows using B-ultrasound machines is as follows: embryos can be seen at 19-24 days, allantois can be seen at 22-25 days, heartbeats can be seen at 19-30 days, forelimbs can be observed at 28-33 days, hindlimbs can be observed at 30-33 days, amniotic membranes can be observed at 30-35 days, spine can be observed at 29-38 days, placenta and appendages can be observed at 33-38 days, detached hooves can be observed at 42-49 days, fetal movements can be observed at 42-50 days, and ribs can be observed at 51-55 days. Pregnancy is established based on the relationship between the embryo and the mother. Multiple factors determine whether pregnancy can be established and maintained, such as endocrine environment, chromosome status, heat stress, nutrition, disease, and many other factors. Embryo-fetal loss can occur at various stages of dairy cow pregnancy, and the highest loss rate is generally after mating, that is, the embryonic stage before the embryo-fetus. Thacher et al. reported that the embryonic loss rates at 0-7 days and 8-11 days of pregnancy were approximately 30% and 400/0, respectively. As pregnancy progresses, pregnancy loss decreases sharply, and the fetal loss rate generally does not exceed 10% after 40 days. The embryo loss rate of Chinese Holstein cows within 30 to 60 days after mating is 25%, and the loss rate within 60 to 90 days is 27.3%. The diameter of the empty horn changes relatively slowly, and the diameter of the pregnant horn changes slowly before 60 days, and increases linearly after 60 days. The diameter difference between the pregnant horn and the empty horn increases rapidly between 60 and 90 days.