Fertilization occurs in the oviducts in both dogs and cats. Implantation of zygotes in the uterus occurs at ~18 days in female dogs and 14 days in the queen. This is accompanied by the formation of small swellings along the uterine horns (deciduomata) by ~21 days. These are palpable, assuming the animal is cooperative, at this time. Fetal growth is rapid during early pregnancy, and these swellings double in diameter every 7 days. After day 35–38, they become less distinct, and palpation becomes difficult until late pregnancy when fetal heads and rumps are palpable as firm, nodular structures in the ventral caudal abdomen. A commercial relaxin assay, specific and sensitive for pregnancy diagnosis in dogs after 30 days gestation, is available.
Although the fetal skeleton begins to calcify as early as day 28, it is not detectable by routine radiography until approximately day 42–45 and is quite prominent by day 47–48. Radiography at this time is not teratogenic. Late gestational radiography (>55 days) is the best method to determine litter size. Fetal dentition becomes visible at term, and its appearance can be used to confirm fetal development adequate for an elective cesarean section when ovulation timing is not available and breeding dates are vague or spread over many days.
Ultrasonography is also useful in pregnancy determination and permits evaluation of fetal viability. Ultrasonography is best performed at 25–35 days gestation. Before 21 days, “false-negative” results are seen. Doppler-type instruments allow one to “hear” the fetal heart, which beats 2–3 times faster than that of the dam. Placental sounds may also be heard. Ultrasonography is especially helpful in differentiating pregnancy from other causes of uterine distention (eg, hydrometra, pyometra, mucometra). Ultrasonographic measurements can be used to calculate gestational age ( see Table: Gestational Age in Dogs and Cats Gestational Age in Dogs and Cats ).
Gestational age in cats can also be determined by the following formula:
mean litter crown rump length (in cm) = 0.2423 × gestational age – 4.2165