The female dog is allowed to mate for a long time, and the ovulation time is difficult to determine. The ovulated eggs still need 2-5 days to complete meiosis, and the sperm survives for a long time in the female dog's reproductive tract (it has been reported that sperm is still active 268 hours after mating), so the gestation period of the female dog is difficult to determine. Calculated from the first mating day, the gestation period ranges from 54 to 72 days, with an average of 63 ± 2 days. In addition, the length of the gestation period is proportional to the parity and inversely proportional to the number of fetuses.

There is no significant difference in the progesterone level between pregnant female dogs and infertile female dogs in late estrus, and late estrus is slightly longer than the gestation period. Therefore, the methods commonly used in livestock to measure progesterone levels or observe whether the next estrus occurs cannot be used to determine whether the female dog is pregnant.

Veterinary Ultrasound diagnosis: Doppler ultrasound probe: Mainly used to detect fetal heart pulses, the accuracy rate of pregnancy diagnosis is 85% at 36-42 days, and can reach 100% after 43 days. If the dog is too small, the abdominal artery sound will interfere with the detection of the uterine artery and umbilical artery, so it is not often used.

Veterinary A-ultrasound scanner: Fetal fluid can be detected at 18-20 days, and the accuracy of pregnancy diagnosis is 90% and the accuracy of infertility diagnosis is 85% at 32-62 days after mating.

veterinary B-ultrasound scanner: More accurate than the previous two methods, the fetus can be found at 18-19 days, the fetus can be counted at 28-35 days, and the fetus can be clearly seen after 40 days.

Veterinary X-ray detection: After injecting 200-800ml CO2 into the peritoneal cavity, the enlarged uterus can be seen at 30-35 days, and the fetal spine and pelvic bones can be seen at 40-49 days. In addition, in the late pregnancy, the fetal heart sound can be auscultated or laparoscopic observation can be used to determine whether it is pregnant. When the embryo is implanted at 19-22 days, the embryo secretes estrogen, and the estrogen concentration in the urine of pregnant female dogs is slightly higher than that of infertile dogs.