Pregnancy diagnosis in dogs tends to be a simple task with an ultrasound. Starting at approximately 28 days, fetuses can be easily detected with transabdominal ultrasound. For large dogs, any probe can be used, however a higher frequency probe is preferred. In small dogs, any probe with a frequency exceeding 5-6 MHz will be adequate.
Position the probe at the brim of the pelvis and begin by scanning forward. The vesicle and fetal structures of each pup will be easily distinguished after 28 days, and the uterine horns can be followed along the dorsal and dorsolateral abdominal walls in lateral or dorsal recumbency. Typically, the pups will be in fairly close proximity, and counting will be easy in mid-gestation. In late gestation, assessment of fetal structures and fetal heart rate will be easier, but counting may be more difficult.
Since dogs have a fairly short and predictable gestation length, and usually a well-known breeding date or dates, gestational aging is often confirmational. Early on (<40 days), if breeding date is unknown, age can be estimated from Crown-Rump Length (CRL) using the formula (3 x CRL) + 27. Later, the transverse diameter of the body at the level of the liver (BD) can be used with the formula (7 x BD) + 29. Subtract these ages from 64 days for an approximate estimation of remaining gestation. Due to substantial breed/size differences, these dates may vary 3-4 days early or late. Use your judgement to aid in estimation, but early on, provide owners with 6-8 day range to account for differences.
One of the advantages to ultrasound is the ability to assess viability of fetuses. After 28 days, one can assess fetal heartrate and morphology of observed structures. While morphology is better covered elsewhere, some examples are easy to visualize (hydrocephalus, fetal loss, amelia, inappropriate limb development, etc). Heartrate is fairly straightforward as well. A normal fetal heartrate should be 2-3 times that of the mother. Elevations beyond that indicate fetal stress and may be associated with proximity to whelping. An abnormally low heartrate is associated with poor fetal viability.
When using a ReproScan ultrasound for canine gestational exams, I prefer to use either a convex T-handle or microconvex probe, however the convex rectal works fine and is shown in the attached images. I prefer to set my gain a little lower and increase my dynamic harmonization (Dyn). Adjust your IP, IE, and Gain to suit your environment. In an exam room with fluorescent lights, the darker screen can help to better differentiate fetal tissue. In a dark room, the gain set closer to default will work just fine. More important is to set your depth and focus appropriately. Remember that unlike our large animal patients, these dogs will often have an abdomen that is 8 to 35 cm in diameter, and the fetuses will be close to the body wall, so you can usually get by with a depth of 10 cm or less, and a focus of 7 cm or less.
Happy Scanning!
- Ruffin Hutchinson DVM ; Shelby, North Carolina