Fetal Count Radiology


Matthew Wright, DVM, MS, Diplomate ACVR

Although fetal count radiographs are valuable when planning whelping, pet owners have questions about the risks from the associated radiation.

Fetal count radiographic procedures are commonly performed in veterinary medicine. These studies take place late in an animal’s pregnancy, and allow the veterinarian or veterinary technician to count the number of fetuses present.

It is often essential for breeders and pet owners to accurately assess the number of fetuses present prior to whelping. An accurate fetal count:

  • Facilitates treatment in cases of fetal retention (dead fetus)
  • Allows for early intervention in cases of dystocia.

Although these procedures are valuable when planning whelping, breeders and pet owners have questioned the safety of this procedure with regard to the risks associated with radiation-induced carcinogenesis.


In human medicine, fetal assessment is done by diagnostic ultrasound. However, the majority of experienced radiologists agree that diagnostic abdominal ultrasound is a poor predictor of fetal counts in species that bear litters.1

Ultrasound is inaccurate for assessing fetal number because:

  • Only a small portion of the uterus is imaged in one scanning plane
  • Fetuses may be counted more than once or not counted at all.

Because of these limitations of ultrasound, radiographic evaluation is the only practical and readily available method of fetal counting in veterinary medicine.2


In order to understand the risks associated with fetal radiation exposure, the veterinarian and veterinary technician must have a working understanding of the amount of radiation used during a fetal count procedure. However, the units used to describe radiation exposure can be confusing.

For this discussion, all units will be converted to millirem (mrem). This unit of radiation exposure is the one most familiar to veterinary team members. As you consider the risks and benefits of fetal count imaging, keep in mind that the exposure from a single lateral radiograph of the abdomen, estimated from exposures described in human medicine, is 30 to 50 mrem.


Mutations & Miscarriage

Noncarcinogenic effects of radiation on the fetus vary with gestational age and radiation dose.

Gestational Age. In humans, embryonic death and congenital malformations most likely occur as a result of radiation exposure during the first 6 weeks of pregnancy. Therefore, in humans, embryonic death typically takes place during the preimplantation period (0-9 days), while malformations usually occur during organogenesis (10 days-6 weeks). Effects on the central nervous system occur early in the fetal period (6 weeks-term), with a peak sensitivity at 8 to 15 weeks.

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[Click Here to Shop .243 Winchester Ammo]What we can do is provide a framework to understand what average conditions might look like, and whether those are reasonably viable for a shot from the average shooter to harvest a moose in the fewest number of shots possible, i.e., ethically. Let’s dive right in. In the question of “Is the .243 Winchester within the ideal range of suitable calibers for moose hunting?” our answer is: No, the .243 Winchester is UNDERKILL for moose hunting, under average conditions, from a mid-range distance, with a medium grain expanding bullet, and with correct shot placement.Let’s look at those assumptions a bit closer in the following table. Assumption Value Caliber .243 Winchester Animal Species Moose Muzzle Energy 1950 foot-pounds Animal Weight 1200 lbs Shot Distance 200 yardsWhat is the average muzzle energy for a .243 Winchester? In this case, we have assumed the average muzzle energy for a .243 Winchester round is approximately 1950 foot-pounds. What is the average weight of an adult male moose? Here we have leaned conservative by taking the average weight of a male individual of the species, since females generally weigh less and require less stopping power. In this case, the average weight of an adult male moose is approximately 1200 lbs. [Click Here to Shop .243 Winchester Ammo]What is the distance this species is typically hunted from? Distance, of course, plays an important role in the viability of a given caliber in moose hunting. The kinetic energy of the projectile drops dramatically the further downrange it travels primarily due to energy lost in the form of heat generated by friction against the air itself. This phenonemon is known as drag or air resistance. Thus, a caliber that is effective from 50 yards may not have enough stopping power from 200 yards. With that said, we have assumed the average hunting distance for moose to be approximately 200 yards. What about the other assumptions? We have three other primary assumptions being made here. First, the average bullet weight is encapsulated in the average muzzle energy for the .243 Winchester. The second important assumption is ‘slightly-suboptimal’ to ‘optimal’ shot placement. That is to say, we assume the moose being harvested is shot directly or nearly directly in the vitals (heart and/or lungs). The third assumption is that a projectile with appropriate terminal ballistics is being used, which for hunting usually means an expanding bullet.Various calibersA common thread you may encounter in online forums is anecdote after anecdote of large animals being brought down by small caliber bullets, or small animals surviving large caliber bullets. Of course those stories exist, and they are not disputed here. A 22LR cartridge can fell a bull elephant under the right conditions, and a newborn squirrel can survive a 50 BMG round under other specific conditions. Again, the goal of this article is simply to address the question of whether .243 Winchester is within the ideal range of suitable calibers to harvest moose - and to this question, the response again is no, the .243 Winchester is UNDERKILL for moose hunting. [Click Here to Shop .243 Winchester Ammo]This article does not serve as the final say, but simply as a starting point for beginner hunters, as well as a venue for further discussion. Please feel free to agree, disagree, and share stories from your own experience in the comments section below. Disclaimer: the information above is purely for illustrative purposes and should not be taken as permission to use a particular caliber, a statement of the legality or safety of using certain calibers, or legal advice in any way. You must read and understand your own local laws before hunting moose to know whether your caliber of choice is a legal option.Foundry Outdoors is your trusted home for buying archery, camping, fishing, hunting, shooting sports, and outdoor gear online.We offer cheap ammo and bulk ammo deals on the most popular ammo calibers. We have a variety of deals on Rifle Ammo, Handgun Ammo, Shotgun Ammo & Rimfire Ammo, as well as ammo for target practice, plinking, hunting, or shooting competitions. Our website lists special deals on 9mm Ammo, 10mm Ammo, 45-70 Ammo, 6.5 Creedmoor ammo, 300 Blackout Ammo, 10mm Ammo, 5.56 Ammo, Underwood Ammo, Buffalo Bore Ammo and more special deals on bulk ammo.We offer a 100% Authenticity Guarantee on all products sold on our website. Please email us if you have questions about any of our product listings. 2 Comments Debbie Tomaganuk - Dec 02, 2020You are absolutely wrong my friend. A 243 will take out moose cleanly with very little tissue damage to parts that are considered edible. 100 grain sp is suitable. David Gregoire - Oct 03, 2023I have a 243 and a 270. To compare both calibers on deer, the 243 will kill…….but my 3 deers shot with it left no blood trail. All 3 deer took off and ran a good 80 yards before dropping. Bullets went right through, but did not have the same knock down power of my 270. My 270 leaves a really good splash of blood at the shot sight, and deer rarely go more than 20 yards before falling. All deer are shot in the vitals. Yes it could probably kill moose, but i believe it is calling for trouble. I say stick to smaller animals with a 243. Leave a commentComments have to be approved before showing up Your Name * Your Email * Your Comment * Post Comment

Radiation Dose. Although we do not have data for canine patients, in human medicine:

  • The Health Physics Society states that the level of radiation exposure used in most diagnostic procedures (< 5 rads or 5000 mrem) will not increase noncarcinogenic reproductive risks (birth defects or miscarriage) during any stage of gestation in humans.3
  • According to the U.S. Nuclear Regulatory Commission, the maximum permissible dose limit to a human embryo/fetus is:
    • 500 mrem within 9 months
    • Maximum of 50 mrem per month.
  • Doses below those outlined above are considered an acceptable radiation exposure dose for a human embryo/fetus.4
  • The Centers for Disease Control and Prevention (CDC) recommend that radiation-induced noncarcinogenic health effects are unlikely in humans, below approximately 0.50 Gy (50 rads or 50,000 mrem) from 16 weeks gestation to birth.5

This information demonstrates that the radiation exposure from a single lateral projection of the abdomen (≈ 30-50 mrem) is far less than 50,000 mrem and, therefore, a fetal count evaluation carries little risk of noncarcinogenic side effects (developmental abnormalities or miscarriages) to the fetus when performed late in pregnancy.


Although no data are available for canine patients, the CDC reports that, in humans, the:5

  • Estimated childhood cancer incidence in patients exposed to radiation during diagnostic radiographic procedures (0-5 rads or 0-5000 mrem) is 0.3% to 1%
  • Estimated lifetime cancer incidence for these same patients is 38% to 40%.

It is important to note that the:

  • Estimated childhood cancer incidence in humans not exposed to radiation (above background radiation levels) is 0.3%
  • Estimated lifetime cancer incidence in humans not exposed to radiation is 38%.

Therefore, human patients exposed to low levels of radiation in utero have only a small increase (< 1%) in the incidence of childhood cancer and small increase (< 2%) in the incidence of lifetime cancer.

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In addition, the high end of the range used to make the above estimates (5000 mrem) far exceeds the exposure from a single lateral radiographic projection of the abdomen (50 mrem). Unfortunately, we do not have data evaluating the cancer risk in dogs (or humans) from a single lateral projection of the abdomen.


Given that all radiation exposure should be considered potentially harmful, a risk/benefit analysis must be performed prior to obtaining a fetal count study, and the benefits of obtaining the study must outweigh the risks associated with fetal exposure.

In veterinary medicine, we do not have a cost effective or efficient method to determine fetal number without using ionizing radiation. As stated previously, diagnostic ultrasound is poor at predicting fetal number in litter-bearing species.

For many breeders and pet owners, the benefit of knowing the number of fetuses prior to whelping outweighs the risk of exposing fetuses to ionizing radiation.


  • Perform a risk/benefit analysis prior to obtaining a fetal count radiographic study.
  • Discuss with clients the risk/benefit analysis of performing a fetal count study prior to obtaining the radiograph
  • Advise clients that:
    • All radiation is potentially harmful
    • Our understanding of the risks associated with fetal radiation exposure is largely based on correlates in other species rather than direct research in canine or feline patients.
  • Limit radiation exposure to the fetuses because all radiation is potentially harmful. Only perform fetal count studies if clinically indicated and at a time during pregnancy the study is most likely to be accurate, which avoids the need to repeat radiographs if the initial set was obtained too early during pregnancy.
  • Advise technicians to only obtain a single lateral projection. A ventral dorsal projection may be necessary in some cases to assess:
    • Fetal health
    • Fetal positioning in relation to the pelvic canal
    • Fetal number if there are difficulties assessing fetal number based on lateral projection.

    However, discourage orthogonal projections and/or repeat projections to obtain perfect positioning or exposure.

  • Obtain fetal count evaluations on or about day 55 after ovulation to help ensure fetal mineralization has progressed, making the fetuses easily identified by radiology.

Older texts advise that fetal mineralization may be identified radiographically after day 41. However, mineralization progresses over the course of several days and there is no way to determine fertilization timing in the dog (it may happen several days after ovulation). Therefore, obtaining radiographs at day 41 is no longer a practical recommendation because a late fertilization will result in incomplete mineralization and failed radiographic detection.

Figure 2. Lateral (A) and ventrodorsal (B) radiographs of a normal pregnancy; note that it is easier to identify the bones in the lateral than the ventrodorsal radiograph.

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Although it cannot be stated unequivocally that there is no risk for increased incidence in lifetime cancer rates as a result of fetal count radiographic evaluations, interpolation of the data from human medicine suggests that, given the levels of radiation used in these evaluations, the risk associated with these studies is low relative to the benefits obtained by the procedure.

CDC = Centers for Disease Control and Prevention; mrem = millirem


  1. Nyland T, Mattoon J. Small Animal Diagnostic Ultrasound. Philadelphia: WB Saunders, 2002.
  2. Toal RL, Walker MA, Henry GA. A comparison of real-time ultrasound, palpation and radiography in pregnancy detection and litter size determination in the bitch. Vet Radiol 1986; 27:102-108.
  3. Brent R. Pregnancy and Radiation Exposure. Available at http://hps.org/hpspublications/articles/ pregnancyandradiationexposureinfosheet.html.
  4. U.S. Nuclear Regulatory Commission. Part 20—Standards for protection against radiation. NRC Regulations. Available at nrc.gov/reading-rm/ doc-collections/cfr/part020/full-text.html.
  5. Radiation and Pregnancy: A Fact Sheet for Clinicians. Available at http://www.bt.cdc.gov/radiation/prenatalphysician.asp.

Suggested Reading

  • Brenner DJ, Doll R, Goodhead DT, et al. Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know. Proc Natl Acad Sci 2003; 100:13761-13766.
  • Doll R, Wakeford R. Risk of childhood cancer from fetal irradiation. Br J Radiol 1997; 70:130-139.
  • Lopate C. Estimation of gestational age and assessment of canine fetal maturation using radiology and ultrasonography: A review. Theriogenol 2008 Aug; 70:397-402.

Matthew Wright, DVM, MS, Diplomate ACVR, is a staff radiologist with Idexx Telemedicine Consultants. He is the author of numerous books, articles, and educational series about radiation safety, including Radiation Safety for the Veterinary Technician. The opinions expressed in this article do not necessarily reflect those of his employer.

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