At the end of the estrus period, the female dog begins to reject the male dog. Based on this, it is believed that the female dog has entered the late estrus period. In animal B-ultrasound, it can be seen that the follicles on the female dog's ovaries have basically disappeared. The length of this period has not yet been determined. If there is no pregnancy, this period generally lasts for 2-3 months. At this time, the concentration of progesterone in the plasma of pregnant female dogs continues to rise, but it drops to the basal concentration at the time of delivery.

Anestrus period, this period is the transition stage from one reproductive cycle to another. The duration varies from 1 to 9 months. This stage is generally manifested in a relatively static state of the ovaries on animal B-ultrasound. When the dog does not show reproductive behavior, it can be considered to be in anestrus. If the dog is pregnant and gives birth normally, the interval between the two estrus cycles of the female dog is about 2 months. Animal B-ultrasound examination can be frequently used at this stage.

The length of dog pregnancy is generally defined as the period from the first natural mating or artificial insemination to the successful delivery, with an average of 62-64 days. The range of gestation time is about 56-72 days, and it is related to many factors, such as the breed, age, parity, litter size, season and body size of the dog. The average litter size of most dog breeds is about 3-7, and the litter size of the smallest and largest breeds varies greatly, ranging from 1 to more than 15. During pregnancy, it is important to use animal B-ultrasound to check the litter size of the female dog for good management. Although the estrus time of different dog breeds is not exactly the same, the entire estrus period shows certain seasonal changes. Due to the high cost of hormone detection methods, most of the domestic methods are still using vaginal exfoliated cell microscopy and saliva examination methods, as well as more accurate animal B-ultrasound diagnosis methods.