Performing ultrasound to evaluate pregnancy (Proceedings)

Normal (Nongravid) Female Reproductive Tract: Bitch and Queen

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 continuous hypoechoic round structure dorsal to the anechoic urinary bladder and ventral to the hyperechoic, crescent shaped colon. Thinking of the urinary bladder as a clock face, the uterine body will be located at 5 o'clock or 7 o'clock. The full urinary bladder acts as an acoustic window to improve imaging the uterus. The cervix is located slightly cranial to the bladder trigone and is best seen when under hormonal influence rather than during anestrus. The cervix is an oblique hyperechoic linear structure in the sagittal view. The uterine body is smaller in diameter than the cervix and usually extends to the cranial one third of the bladder. The bifurcation of the uterus into the uterine horns can sometimes be imaged; the horns are typically difficult to image unless enlarged due to hormonal influence during the estrous cycle, pregnancy, or from pathology. The uterus is composed of three layers: the mucosa, the muscularis and the serosa. The endometrium and myometrium cannot usually be differentiated in the normal state. The uterine lumen is generally not seen, although it may be visible as a bright echogenic central area, representing a small amount of intraluminal mucus, or as a hypoechoic to anechoic region if fluid is present.

The normal ovaries are located caudal and slightly lateral to the caudal poles of the ipsilateral kidneys. Their location can be facilitated by the appearance of the artifactual distal enhancement dorsal to each ovary. The appearance of the ovaries varies with stages of the estrous cycle. Normal ovarian dimensions have been established for average sized dogs. During anestrus, the ovaries appear as small oval to bean shaped structures with a homogenous echogenicity similar to the renal cortex. The cortex and medulla are not usually differentiated in the bitch and queen. Multiple anechoic or hypoechoic cyst like structures can be visualized in the ovarian parenchyma during folliculogenesis, larger cystic structures are present during the luteal phase.

Pregnancy Diagnosis

Pregnancy detection by abdominal palpation (at approximately 30 gestational days) or radiography (43-46+ days post LH peak, the later the better) can confirm the presence of fetuses at these points in time. Prior to fetal skeletal mineralization, other causes for uterine enlargement (hydrometra, pyometra) cannot be ruled out radiographically. Radiography cannot be used to assess fetal viability in a timely fashion. Once profound post mortem changes have occurred, radiography can detect intra-fetal gas accumulation or abnormal skeletal arrangement suggesting fetal death. Early fetal resorption cannot be detected radiographically, only by ultrasound. Ultrasound is the best method to evaluate early in gestation for pregnancy ("yes or no", fetal viability, litter size, and gestational age).

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 continuous hypoechoic round structure dorsal to the anechoic urinary bladder and ventral to the hyperechoic, crescent shaped colon. Thinking of the urinary bladder as a clock face, the uterine body will be located at 5 o'clock or 7 o'clock. The full urinary bladder acts as an acoustic window to improve imaging the uterus. The cervix is located slightly cranial to the bladder trigone and is best seen when under hormonal influence rather than during anestrus. The cervix is an oblique hyperechoic linear structure in the sagittal view. The uterine body is smaller in diameter than the cervix and usually extends to the cranial one third of the bladder. The bifurcation of the uterus into the uterine horns can sometimes be imaged; the horns are typically difficult to image unless enlarged due to hormonal influence during the estrous cycle, pregnancy, or from pathology. The uterus is composed of three layers: the mucosa, the muscularis and the serosa. The endometrium and myometrium cannot usually be differentiated in the normal state. The uterine lumen is generally not seen in the non pregnant state, although it may be visible as a bright echogenic central area, representing a small amount of intraluminal mucus, or as a hypoechoic to anechoic region if fluid is present.

Start the pregnancy check scan by transversely locating the urinary bladder, colon and uterine body as described earlier. Once the uterine body has been identified, sweep cranially, still in transverse, towards each kidney. You are trying to follow each uterine horn to its ovary. Each fetus is contained in an oval, fluid filled gestational sac. Then answer your 3 questions! First question; Yay or nay? Then, if she is pregnant, viability. Finally, how many are in there? Finally, what is the gestational age? As for when, I like to perform the scan at 30-32 days post the last known breeding; for 3 reasons; First at 30-32 days post the vesicle is larger than transverse small bowel, making identification pretty easy. Second, the flicker of the heartbeat is regularly seen even without Doppler. Lastly, the vesicles are small enough to allow easy counting of fetuses within each uterine horn. In later gestation (>50 days) the fetuses are so large the uterine horns overlap making the correct count difficult.

The determination of gestational age can be of vital importance. An accurate determination of gestational length can be difficult, especially if numerous copulations occurred and no ovulation timing was performed. Prolonged gestation is a form of dystocia. Gestation in the bitch is more challenging to calculate than in the cat, because bitches are spontaneous ovulators. Normal gestation in the bitch is 56 to 58 days from the fist day of diestrus (detected by serial vaginal cytologies, defined as the first day that cytology returns to <50% cornified/superficial cells), 64 to 66 days from the initial rise in progesterone from baseline (generally >2ng/ml), or 58 to 72 days from the first instance that the bitch permitted breeding. Predicting gestational length without prior ovulation timing is difficult because of the disparity between estrual behavior and the actual time of conception in the bitch, and the length of time semen can remain viable in the bitch reproductive tract (often up to >7 days). Breeding dates and conception dates do not correlate closely enough to permit very accurate prediction of whelping dates. Additionally, clinical signs of term pregnancy are not specific: radiographic appearance of fetal skeletal mineralization varies at term, fetal size varies with breed and litter size, and the characteristic drop in body temperature (typically less than 99 degrees Fahrenheit) may not be detected in all bitches and varies in many. Breed, parity and litter size can also influence gestational length.

Because the queen is an induced ovulator (ovulation follows coitus by 24-36 hours), gestational length can be predicted more accurately from breeding dates, assuming copulation provided adequate coital stimulation for the LH surge and subsequent ovulation, and a limited number of copulations were permitted. The gestational length of queens ranges from 52-74 days from the first to last breeding. The mean gestational length is 65-66 days. Because of the poor outcome with the delivery of premature puppies and kittens, elective intervention is best delayed until stage I labor has begun, or prolonged gestation confirmed.

Definite ultrasonographic diagnosis of pregnancy in the queen based on the appearance of a "fetal pole" can be made at 15-17 days post coitus, although gravid uterine enlargement (4-14 days) and the presence of a gestational sac (11-14 days) can be detected even earlier. Ultrasonographic detection of the canine blastocyst (a 2-3 millimeter spherical hypoechoic structure surrounded by a hyperechoic rim within the uterus) occurs at 19-20 days post LH peak. Ultrasonography permits evaluation of early fetal cardiac motion (21-22 days post LH peak), fetal movement (31-32 days post LH peak) and the fetal heart rate, enabling assessment of viability. By 30 days gestation (30 days after the LH surge) pregnancy diagnosis with ultrasonography is straightforward.

Fetal age determination by ultrasonography is accomplished in two ways: 1. the first appearance of visible structures and 2. the measurement of certain parameters. Predicting fetal age by noting the first appearance of visible structures corresponding to gestational length is often more accurate than measurements. Measurements such as the gestational sac diameter, fetal occipitosacral (crown-rump) length and fetal skull (biparietal) diameter can be obtained ultrasonographically, relate closely to fetal age, and permit estimation of gestational length and parturition dates, especially useful if ovulation timing was not performed. Variation in breed size (especially in the dog) and individual variation in measuring technique are sources of inaccuracy in predicting fetal age with ultrasound. Ultrasonography is less accurate than radiography in estimating litter size, particularly later in gestation, due to its dynamic nature. Keep in mind that fetal resorption or abortion can alter litter size after early ultrasound estimates are made.

Table 1

Formulas to Predict Gestational Age and Days before Parturition in the Dog and Cat Gestational age (GA) is based on days post luteinizing hormone (LH) surge in the dog and days post breeding in the cat. Gestational sac diameter (GSD), crown-rump length (CRL), head diameter (HD), and body diameter (BD) measurements are in centimeters. Days before parturition (DBP) is based on 65 ± 1 days post LH surge in the dog and 61 days post breeding in the cat. Data modified from Nyland et al.

Pregnancy Termination:

Purposeful induction of abortion with drugs (prostaglandins, dexamethasone, antiprogestins) also requires serial ultrasonographic evaluations to determine the endpoint of therapy, as most protocols will result in delivery of viable neonates if terminated prematurely. Loss of fetal viability usually precedes expulsion of fetuses by 12-48 hours.