Fertilization occurs in the oviducts in both the bitch and queen. Implantation of zygotes in the uterus occurs at ~18 days in the bitch 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 indistinct, and palpation becomes difficult until late pregnancy when fetal heads and rumps are palpable as firm, nodular structures in the ventral posterior abdomen. A commercial relaxin assay, specific and sensitive for pregnancy diagnosis in the bitch 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 about day 42–45 and is quite prominent by day 47–48. Radiography at this time is not teratogenic. Late gestational radiography is the best method to determine litter size.

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).

Last full review/revision July 2011 by Autumn P. Davidson, DVM, MS, DACVIM