Pregnancy diagnosis in the dog is complicated by the fact that all dogs go through the same sequence of hormone changes after estrus, whether or not they are bred. For that reason, blood tests are not as commonly used to diagnose pregnancy as are diagnostic methods that physically identify pregnancy. The various methods of pregnancy diagnosis that are commonly used vary in time during gestation when they can be used, accuracy, and ability to provide information beyond a simple “yes or no” answer (Table Methods for Pregnancy Diagnosis in Dogs).
Table Methods for Pregnancy Diagnosis in Dogs
|Method||Days After Ovulation When Accurate||Accuracy
|Useful for Determination of Litter Size?||Useful for Determination of Fetal Viability?|
|Abdominal palpation||28-35||88%||No. Most commonly err by palpating feces in the colon and identifying it as a pup||No. Pups may be seen moving in late gestation|
|Ultrasound||>24||100%||Somewhat: Most useful for litters of 5 or fewer, at mid-gestation||Yes. Best technique for assessment of fetal viability and fetal stress|
|Radiographs||>42-45||100%||Yes. Best technique in very late gestation||Somewhat. If pups have been dead for at least 24 hr, signs of fetal death may be evident|
|Relaxin assay||>28-30 or more||Undefined||No. Relaxin concentrations are not correlated with litter size||No. Relaxin concentrations remain high for an undetermined length of time after pregnancy loss|
Human “early pregnancy tests” do not work in dogs. These tests identify the hormone human chorionic gonadotropin (hCG), which is produced only during pregnancy in humans. The human conceptus implants 6 days after conception; so, by the time a woman suspects she may be pregnant, 14 days after ovulation, there has been a functional placenta secreting hCG for about 8 days. No hormone of this type is known to be produced in dogs during pregnancy. Because dogs do not develop a functional placenta until after implantation on about day 17 after ovulation, creation of an early pregnancy test for dogs is not likely.
Palpation of the abdomen permits the experienced individual to feel amniotic vesicles. The amniotic vesicles are the developing pups within the amniotic membranes, surrounded by fluid. These turgid vesicles have the consistency of peeled, hard-boiled eggs and range in size at midgestation from a grape in very small dogs to an egg in large-breed dogs. The vesicles are not grasped but instead are allowed to slip through the fingers as the abdominal contents pass through the fingers (). Abdominal palpation never has been documented to cause pregnancy loss in bitches, but repeated palpation day after day is not recommended.
Abdominal palpation is best performed from about days 28 to 35 from ovulation. Amniotic vesicles are too small to feel much earlier than 28 days from ovulation. After about 35 days from ovulation, the amniotic vesicles fill with fluid and abut each other end to end, making them difficult to feel as individual entities. It is difficult to diagnose pregnancy in bitches that are tense, very thin, or very obese. Overall accuracy is reported to be 88%. If a mistake is made, it is more often a false-positive; feces in the colon can feel very much like amniotic vesicles, as can a segmented uterus affected with pyometra ().
Abdominal palpation is not at all accurate as an indicator of litter size. Some of the abdominal space lies within the rib cage and cannot be palpated. Similarly, abdominal palpation is not an accurate indicator of fetal viability unless the pregnancy is very advanced and the person palpating can feel or see the pups kicking.
Several types of ultrasound are described in the veterinary literature. Those used commonly in small animal medicine are Doppler and B-mode ultrasound. With any type of ultrasound, sound waves are sent into tissue and reflected back. The reflection may be audible, as in Doppler ultrasound, or visible, as with B-mode ultrasound. Doppler ultrasound is rarely used as a diagnostic tool in veterinary medicine but is a component of systems designed for management of high risk pregnancies and possible dystocias (WhelpWise, Whelp Watch). For pregnancy diagnosis, B-mode ultrasound most commonly is used ().
Amniotic vesicles easily are identified by B-mode ultrasound. The amniotic vesicles are balls filled with fluid, with a tiny developing puppy in the center. Fluid appears black on ultrasound, so B-mode ultrasound of a pregnant bitch’s uterus reveals multiple black spheres with gray material inside (). There are reports of identification of pregnancy by B-mode ultrasound as early as 19 days after ovulation, but these amniotic vesicles are small and easily missed. Once the bitch is 24 days or more past ovulation, B-mode ultrasound is 94% to 98% accurate for pregnancy diagnosis.
B-mode ultrasound is not the most accurate technique for determination of litter size, with reported accuracy of 32% to 36%. Because only a portion of the abdomen is being viewed at a time and the uterine horns can wind from side to side of the abdomen, it is easy to count a specific puppy more than once. Accuracy for determination of litter size is greater in litters of five or less because the puppies are spaced farther apart in the abdomen.
Ultrasound is the best technique available for determination of fetal viability. B-mode ultrasound is “real time”; beating hearts and movement of limbs readily are identified after midgestation. Fetal heart rate also can be used as an indicator of fetal distress. Fetal heart rate should be about twice that of the dam, usually greater than 200 beats per minute. Fetal heart rates of less than 150 beats per minute signal severe fetal distress and the need for immediate veterinary intervention.
Radiographs, or x-rays, are not useful for pregnancy diagnosis until the fetuses start to mineralize. The pups are completely formed well before mineralization of bone occurs. During this time, the enlarged uterus may be visible on radiographs, but simple enlargement of the uterus cannot differentiate pregnancy from pyometra, which also occurs at this stage of the cycle (). Mineralization begins at about day 42 to 45 from ovulation and progresses from the center of the puppy outward. Little is gained by performing radiographs too early. From about day 45 to 50, only scattered pockets of mineralization are visible, and neither the number nor the size of the pups can be determined with any accuracy. From days 50 to term, rapid progressive mineralization occurs. Radiographs taken in the final week before whelping is expected yield excellent information (). They are 100% accurate for pregnancy diagnosis, give excellent information about litter size unless the litter is very large (i.e., more than nine puppies in most breeds), allow general prediction of whelping date (), and give some information about fetal viability. Signs of fetal death that may be seen on radiographs include collapse of the skeleton, overlap of the bones of the skull, gas within or around the fetus, and failure of the fetus to grow or calcify.
A commercial in-house relaxin assay is available for use in veterinary clinics (Witness). The test originally was claimed to be accurate for pregnancy diagnosis as early as 21 days postbreeding but has since been demonstrated to be most accurate if used 28 or more days after breeding. A simple yes or no answer is generated (10-6). There is no correlation between time at which the test becomes positive and litter size. It is not known how long after a pregnancy loss relaxin concentrations remain high in blood, so the kit occasionally yields false-positive results in bitches that resorbed or aborted a pregnancy.
Progesterone concentrations cannot reliably be used to determine pregnancy status in bitches. Average progesterone concentrations are higher in pregnant than in nonpregnant bitches, and progesterone concentrations fall more quickly during diestrus in nonpregnant than in pregnant bitches, but there is great overlap in progesterone values between the two groups. No single value of progesterone is indicative of pregnancy in dogs.
Prolactin concentrations are higher in pregnant than in nonpregnant dogs after midgestation. One study reported that a rapid rise in prolactin could be stimulated in dogs very early in pregnancy by administration of the drug naloxone. Unfortunately no canine prolactin assay is available at this time.
Fibrinogen is a protein released during inflammatory events, such as implantation and placenta formation. There are several reports of the measurement of fibrinogen in blood as a pregnancy diagnostic test in bitches. Fibrinogen also is elevated in bitches with pyometra, which also occurs during this stage of the estrous cycle. Measurement of fibrinogen is not routinely used for pregnancy testing in dogs.