Fertilization occurs in the oviducts in both bitches and queens.
Implantation of zygotes in the uterus occurs at approximately 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 approximately 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 the detection of the presence of placentas (assuming pregnancy) in bitches after 22–27 days gestation, is available.
Courtesy of Dr. Autumn Davidson.
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 (see fetal dentition radiograph).
Gestational Age in Dogs and Cats From Ultrasonographic Measurements
Dogs: Gestational Age ± 3 Days | Cats: Gestational Age ± 2 Days | |
|---|---|---|
< 40 days | > 40 days | > 40 days |
(6 × GSD) + 20 | (15 × BP) + 20 | 25 × BP + 3 |
(3 × CRL) + 27 | (7 × BD) + 29 | 11 × BD + 21 |
NA | (6 × HD) + (3 × BD) + 30 | NA |
Days before parturition: 64–66 minus gestational age | Days before parturition: 61 minus gestational age | |
Abbreviations: BD, body diameter at the liver, cm; BP, biparietal diameter, cm; CRL, crown rump length, cm; GSD, gestational sac diameter, cm; NA, not applicable. | ||
Ultrasonography is most useful in pregnancy determination and permits evaluation of fetal viability. The procedure is best performed at 25–35 days gestation. Before 21 days, false-negative results may occur.
Doppler-type instruments allow one to “hear” the fetal heart, which beats 2–3 times faster than that of the dam.
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 the table Gestational Age in Dogs and Cats From Ultrasonographic Measurements).
Gestational age (in days) in queens can be determined from measured mean litter crown rump length according to the following formula:
mean litter crown rump length (in cm) = 0.2423 × gestational age – 4.2165
