Page 979 - Small Animal Internal Medicine, 6th Edition

CHAPTER 54 The Practice of Theriogenology 951

actual time of conception in the bitch, and the length of time elective intervention is best delayed until stage I labor has
semen can remain viable in the bitch reproductive tract begun (see Chapter 55) or prolonged gestation is confirmed
VetBooks.ir (often > 7 days). Breeding dates and conception dates do not ultrasonographically.
Fetal age determination by ultrasonography is accom-
correlate closely enough to permit very accurate prediction
of whelping dates, and clinical signs of term pregnancy are
tures or by measuring certain parameters. Measurements
not specific. Radiographic appearance of fetal skeletal min- plished by either the first appearance of certain visible struc-
eralization varies at term, and fetal size varies with breed and such as the gestational sac diameter, fetal occipitosacral
litter size. Because the queen is an induced ovulator (ovula- (crown-rump) length, and fetal head (biparietal) diameter
tion follows coitus by 24-36 hours), gestational length can be can be obtained ultrasonographically, relate closely to fetal
predicted more accurately from breeding dates, assuming age, and permit estimation of gestational length and parturi-
copulation provided adequate coital stimulation for the LH tion dates, especially useful if ovulation timing was not per-
surge and subsequent ovulation, and a limited number of formed (Figs. 54.27, 54.29, 54.30; Video 54.4). Variation in
copulations were permitted. The gestational length of queens breed size (especially in the dog), brachycephalic canine and
ranges from 52 to 74 days from the first to last breeding. The feline breed variation, and individual variation in measuring
mean gestational length is 65 to 66 days. Because of the poor technique are sources of inaccuracy in predicting fetal age
outcome with the delivery of premature puppies and kittens, with ultrasound (Box 54.6).

0
0
1

1 2

3
2
4
1.92cm 1.22cm

FIG 54.29 FIG 54.30
Pregnancy ultrasound feline; gestational age 30 days. Cursors Pregnancy ultrasound canine; gestational age 39 days.
mark the occipitosacral (crown-rump) length in centimeters. Cursors mark the biparietal (head) diameter in centimeters.

BOX 54.6
Formulas to Predict Gestational Age and Days Before Parturition in the Dog and Cat

Gestational age (GA) is based on days post–luteinizing Days before parturition in the dog
hormone (LH) surge in the dog and days postbreeding in the DBP = 65 − GA
cat. Gestational sac diameter (GSD), crown-rump length
(CRL), head diameter (HD), and body diameter (BD) mea- Gestational Age in the Cat (±2 Days)
surements are in centimeters. Days before parturition (DBP) Greater than 40 days
is based on 65 ± 1 days post-LH surge in the dog and 61 GA = 25 × HD + 3
days postbreeding in the cat. GA = 11 × BD + 21

Gestational Age in the Dog (±3 Days) Days before parturition in the cat
Less than 40 days
DBP = 61 − GA
GA = (6 × GSD) + 20
GA = (3 × CRL) + 27 Novel feline gestational aging using crown-rump length
Y = 0.2423 × GA − 4.2165
Greater than 40 days
Y is mean litter CRL (cm)
GA = (15 × HD) + 20 GA is the gestational age
GA = (7 × BD) + 29 (Solve for GA)
GA = (6 × HD) + (3 × BD) + 30
Data modified from Nyland et al: Small animal diagnostic ultrasound, ed 2, Philadelphia, 2002, Saunders.