Ultrasound is an excellent way to determine if an animal is pregnant, if the fetuses are viable and what the gestational age is. Ultrasound generally is not an accurate method of determining litter size. Fasting the patient before the pregnancy check ultrasound exam is often not necessary. Breeders should be informed ahead of time that the animal will be shaved, as some will object to this. The exam can be started from the caudal midabdomen, imaging the urinary bladder first, then examining dorsally for the uterine body. The left uterine horn is followed cranially, and then the right uterine horn is followed caudally. The uterine horns bend in multiple directions rather than lie in a straight line when a large litter is present, and fetuses can be seen literally ‘everywhere’ in the abdomen. In this situation, a systematic approach to view every fetus, but each fetus only once becomes challenging.
Ultrasound is a very sensitive and specific imaging method to diagnose pregnancy if it is performed at least 30 days after breeding in the dog and at least 16 days after breeding in the cat. Pregnancy has been diagnosed as early as 10 days after breeding in the dog and 11 days after breeding in the cat. False negative exams can occur due to overlooking a fetus, which could happen due to gas or fecal material creating shadows. False positives could occur due to viewing a loop of small intestine, but most of the time imaging in a second plane reveals that intestine becomes tubular, whereas a gestational sac does not. Visualization of a gestational sac is considered to confirm pregnancy. Uterine enlargement will precede the formation of the gestational sac, but that is not a specific finding. Cardiac activity and fetal movement are predictable signs of fetal viability. Visualization of the embryo and cardiac activity usually occur on the same day. Initially, cardiac activity appears as a flutter within the embryo tissue. The fetal heart rate is usually twice the maternal heart rate, and usually is between 200 and 220 beats per minute (bpm). Fetal activity can include swallowing, hiccoughs, and body and limb movements.
Prediction of gestational age can be acco.ukplished by assessing for the presence of fetal structures (See Tables 1 and 2) as well as measuring the gestation sac (GSD), crown-rump length (CRL), and head diameter (HD) and body diameters (BD) of the fetus (See Tables 3 and 4). The timeline of appearance of fetal structures is constant between breeds, and these observations should always be used along with the measurements that are taken to most accurately predict gestational age. When obtaining measurements, at least two fetuses should be used. In dogs, the accuracy of measurements (using GSD, CRL or HD) is best at 30 days post LH peak. The accuracy is not affected by litter size, but in toy and miniature breeds (? 9 kg), one day should be added to the calculated gestational age. In giant breeds (> 40 kg), two days should be subtracted from the calculated age. In the last 3 weeks of gestation, the accuracy falls to about 50%, and the head diameter measurement should be used as it is the most reliable. Overall, the accuracy of prediction of gestational age at any time during gestation, once adjusting for body weight as described above, is 87% ± 2 days from the predicted age.
A reason for this could be that only a small section of the reproductive tract can be imaged at one time, allowing fetuses to be counted more than once or not at all. Fetal resorption may also be a factor. The tendency is to overestimate the size of small litters, and underestimate the size of large litters. One can probably say it is a ‘small litter’ or a ‘large litter’, but otherwise the accuracy is very poor (one study quotes 18% accurate when the exam is done 30-50 days post-breeding, and only 8% when done 50 days post breeding). Many veterinarians and clients are not aware of this limitation of ultrasound, thus they are expecting to get a fetal number along with the diagnosis of pregnancy and prediction of gestational age. The statement, “The litter size will probably be whatever is counted, plus or minus 5”, seems to get the point across that ultrasound is not an accurate way of predicting fetal number.
When a fetus dies before 25-35 days post ovulation, resorption usually occurs. The resorption rate of the entire litter has been reported to be 11% in dogs. Approximately 5-13% of dogs may resorb one or more fetuses, but carry the rest to term. When fetuses die after 35 days, they are usually aborted. Signs of fetal death include increased echogenicity of the embryonic fluid, loss of cardiac activity, loss of fetal activity, loss of detail of fetal structures, collapse of the conceptus and uterine wall and intrauterine or fetal gas (not to be confused with overlying intestinal gas).
Ultrasound can be used to monitor fetal development. The canine embryo grows 1 mm per day from 17 to 30 days post LH peak, after which the growth is exponential. If growth is slow, or development of a fetal structure is not observed 2 or more days after it is expected, the fetus has a greater chance of being resorbed or aborted. Fetal congenital abnormalities have been infrequently reported in veterinary medicine. Fetal stress due to hypoxia, such as might occur during dystocia, is manifested as a slowing of the heart rate. Fetal distress is severe if the heart rate drops below 180 bpm.
The ultrasonographic appearance of the post partum uterus has been described, but evaluation of uterine involution is rarely indicated in a standard practice. The appearance of a retained placenta has not been described, and the placenta would most likely blend into the uterine wall and other luminal contents. Sub-involution of placental sites occurs in the dog, but the ultrasonographic appearance of this has not been described.
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