The earliest time for the gestational sac of many breeds of female dogs to appear under veterinary B-ultrasound conditions is 23 days after breeding, which is related to species and individual differences. The gestational sac develops fastest in the first week that can be detected on veterinary B-ultrasound. It changes extremely significantly every day, rapidly increases in size, and the embryonic cells proliferate rapidly. Within two days, all functional areas of the embryo are initially visible. In the observation of veterinary B-ultrasound, the early developmental changes of the fetus are more significant. In the later observation, except for the increase in fetal size and the gradual calcification of the bones, resulting in the formation of acoustic shadows, other changes are not obvious. In veterinary B-ultrasound and color ultrasound exploration, the uterine changes are not obvious in the early stages of pregnancy. Only the uterine arteries and veins advancing along the uterus can be observed. There are no detectable blood vessels in the uterus. When the gestational sac appears, the blood vessel distribution in the uterus gradually changes. Abundant, especially the development of the placental part.

When an embryonic spot appears in the gestational sac of a female dog, small blood vessels can be seen where the embryonic spot contacts the inner wall of the gestational sac. As the embryonic spot develops and an embryo is formed, after the differentiation of the cardiomyocytes is completed, the cardiomyocytes begin to beat. And the frequency is above 200. Some people have found that veterinary B-ultrasound shows that 30 days after mating, a regular and fast flashing light spot can be seen in the body of the embryo, and they think it is a reaction of the structure and heartbeat. At 38 days of embryonic development, it can be seen that the basic large blood vessel network in the embryo has been formed, and as the fetus develops, the secondary blood vessels in the fetus begin to branch and develop. When observed on the 55th day of pregnancy, the vascular network in the fetus is basically fully developed, indicating that the fetal circulatory system is fully developed. This can provide some reference for judging whether the cardiovascular system is well developed at each stage of fetal development, and can also provide some basis for judging the developmental period of the fetus.