Page 978 - Small Animal Internal Medicine, 6th Edition

950 PART VIII Reproductive System Disorders

approximately 30 gestational days) confirms the presence
of an enlarged, presumably gravid uterus, but gives little
VetBooks.ir other information. Produced by the placenta, the hormone 0
relaxin is elevated in both canine and feline pregnancies.
A serum relaxin assay (Witness Relaxin Assay [Synbiotics/
Pfizer]) is available for pregnancy diagnosis after about
25 to 31 days of gestation, but it can be falsely negative 1
with small litter size. Radiography (55+ days of gestation,
the later the better) can confirm the presence of fetuses.
Before fetal skeletal mineralization, other causes for uterine
enlargement (hydrometra, mucometra, pyometra) cannot 2
be ruled out radiographically. Radiography cannot be used
to assess fetal viability in a timely fashion. Once profound
postmortem changes have occurred, radiography can detect
intrafetal gas accumulation or abnormal skeletal arrange-
ment, suggesting fetal death. A single lateral projection is
preferred; counting both fetal spines and fetal skulls most FIG 54.27
accurately provides litter size. A ventrodorsal projection Early pregnancy ultrasound, feline; gestational age 18
days. Cursors mark the gestational sac diameter in
should only be made if the litter size is difficult to determine centimeters. The fetal pole is evident at 7:00.
on the lateral view, or if fetal positioning in relation to the
pelvic canal needs evaluation at term. A fetal count radio-
graph carries little risk of teratogenic results in the fetuses
(30-50 mrem).
Ultrasound is the optimal method to evaluate for concep-
tion, fetal health, and litter size and is best performed around 0
30 days after breeding (~35 days gestation). In later gestation
(>50 days) the fetuses are so large the uterine horns overlap, 1
making an accurate litter size evaluation difficult. Serial
pregnancy ultrasound can identify resorption and early fetal 2
death, as well as intrauterine pathology. Transabdominal
pregnancy ultrasound is performed with the bitch or queen
in comfortable, padded dorsal recumbency and does not 3
require sedation or clipping. A variable frequency scanhead
(6.0-8.0 MHz) typical of most small animal clinics is ade- 4
quate. Doppler is not required. The normal uterus is best
located by scanning transversely between the urinary bladder
and the colon. The cervix and uterine body are seen as a FIG 54.28
continuous hypoechoic round structure dorsal to the Pregnancy ultrasound canine; gestational age 20 days.
anechoic urinary bladder and ventral to the hyperechoic
crescent-shaped colon. A full urinary bladder acts as an
acoustic window to improve uterine imaging. Once the GESTATIONAL LENGTH AND FETAL
uterine body has been identified, the uterine horns can be AGE DETERMINATION
located by scanning in transverse toward each kidney. Defi- Determining gestational age can be of vital importance if an
nite ultrasonographic diagnosis of pregnancy in the queen elective cesarean section is planned or prolonged gestation
based on the appearance of a “fetal pole” (the actual fetus) suspected. Accurate determination of gestational length can
can be made at 15 to 17 days postcoitus, although gravid be difficult, especially if numerous copulations occurred and
uterine enlargement (4-14 days) and the presence of a ges- no ovulation timing was performed. Prolonged gestation is
tational sac (11-14 days) can be detected even earlier (Fig. a form of dystocia. Gestation in the bitch is more challenging
54.27). Ultrasonographic detection of the canine blastocyst to calculate than in the cat, because bitches are spontaneous
(a 2-3 mm spherical hypoechoic structure surrounded by a ovulators. Normal gestation in the bitch is 56 to 58 days from
hyperechoic rim within the uterus containing the fetus) the first day of diestrus, 64 to 66 days from the initial rise in
occurs at 19 to 20 days post-LH surge (Fig. 54.28). Ultraso- progesterone from baseline, or 64 to 66 days from the LH
nography permits evaluation of early fetal cardiac motion surge. Normal gestation can actually be 58 to 72 days from
(21-22 days post-LH surge), fetal movement (31-32 days the first instance the bitch permitted breeding. Breed and
post-LH surge), and the fetal heart rate, enabling assessment litter size can influence gestational length. Predicting gesta-
of viability. By 30 days’ gestation pregnancy diagnosis with tional length without prior ovulation timing is difficult
ultrasonography is straightforward. because of the disparity between estrual behavior and the