Page 978 - Small Animal Internal Medicine, 6th Edition
P. 978

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
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