Fetal Count Radiography In Dogs And Cats
Fetal Count Radiography is a commonly-performed procedure that allows both veterinarian and pet owner to know how many puppies or kittens are expected in a pregnancy, whether planned or accidental. The X-rays are usually taken late in a pregnancy, after bones in the offspring are sufficiently developed to easily show on the film’s image.
The natural question arises, “Is it safe?” After all, pregnant women are warned to stay away from sources of radiation during their pregnancies.
And, “Ultrasound is used in people to assess a fetus. Why not in pets?”
Let’s answer the second question first, with the help of Dr. Matthew Wright, D.V.M., MS, Diplomate of the American College of Veterinary Radiology. Dr. Wright addressed this issue in the November/December Today’s Veterinary Practice, a peer-reviewed journal.
The main problem with ultrasound is fetal count accuracy, Dr. Wright says.
The human uterus is essentially one compartment, whereas the canine uterus and feline uterus are elongated, with two long tubes called “uterine horns.”
“Only a small portion of the uterus is imaged in one scanning plane,” in ultrasound. Therefore, “fetuses may be counted more than once or not counted at all,” Dr. Wright opines.
With radiography, the entire uterus may be seen at one time. Granted, there may be the occasional large-breed dog whose uterus extends beyond the 14″x17″ size limit of the largest X-ray film, but, since radiography is static, landmarks can be identified to specify the location of each fetus with sufficient accuracy so that the two films can be compared and an accurate count can result.
SAFETY–WHAT ABOUT BIRTH DEFECTS?
While most human couples insist on zero exposure to radiation during pregnancy, the U. S. Nuclear Regulatory Commission (USNRC) has established safe levels for human embryo/fetus exposure. Danger level changes with the age of any fetus, and, for dogs, waiting until at least 55 days gestation allows the canine fetus to be well-developed before the first X-ray is taken. Dr. Wright points out that a single radiograph of a female dog’s abdomen is approximately 1/1000th of the accepted safe level of radiation for a human fetus, in terms of radiation-induced mutations. Such information has not been collected for canine or feline patients, leaving us to extrapolate from human data.
WHAT ABOUT CANCER?
Childhood cancer rates for humans never exposed to radiation (excepting so-called “background radiation” from the sun and environment) is about 0.3%. For those radiographed, the rate rises, but only to 1%.
No medical procedure is ever undertaken without consideration of the risks and the advantages to the patient. A radiograph would never be taken unless there was a potential for a problem, such as a female who has had previous difficult deliveries, failure to pass puppies, etc. In the case of accidental breeding, a large male dog coupled with a small female poses a great risk of complications.
Rarely is there justification for multiple radiographs. Radiation exposure should be held to a minimum, which is why Dr. Wright suggests waiting until 55 days for the first radiograph. Some old literature indicated that the bones of fetuses were sufficiently mineralized as early as 41 days. Even with a known breeding date it is impossible to know when fertilization occurred, which is the determining factor in mineralization progress.
Usually a lateral (from the side) projection gives the most information. Under special circumstances, a dorsoventral (bottom to top) view may be required for a more accurate count of skeletons lying close to one another, or to evaluate a possible deformation, Dr. Wright says.
Taking those risks into consideration, and knowing no one will X-ray a female dog unnecessarily, each situation is weighed individually.
Happy New Year, Dr. Randolph.