Updated: Feb 29, 2020
Hariamrit Khalsa contributed to this post.
Common sense disclaimer: As with everything else on this blog, it’s critical to seek the advice of a qualified veterinarian, preferably one that is board certified in theriogenology (reproductive science) for reproductive matters. This website, its blog, and its courses are NOT designed nor intended to replace the need for a qualified veterinarian, but instead to help educate people to to work optimally with their veterinarians. All recommendations should be reviewed with qualified professionals, such as a board certified reproductive veterinarian, prior to implementation in a breeding program. Always seek the advice of your veterinarian. Readers assume all risks associated with use of material on this site. More here.
The purpose of this post is NOT to tell you whether you should x-ray for puppy count. That’s a decision you should be making in discussion with your reproductive specialist, based on breed, risk for each individual dog, and preference. The material presented here is to help you decide for your own program and dogs.
There are a few issues breeders debate regularly. The utility and safety of pre-whelping x-rays is one of them.
The two common reasons to take x-rays before whelping are:
Puppy count so breeders can know when a dog has finished whelping.
Avoidance of dystocia—A stuck or retained puppy can endanger the dam and any unborn puppies. X-rays can’t always identify dead or deformed puppies, but often can when read by a skilled radiologist. For some breeds, in particular, having a radiologist measure skulls and consulting with a reproductive vet will also help you determine if it’s likely puppies can fit through the birth canal. If a problem is detected on the x-ray then a breeder and vet can intervene early.
While ultrasound is useful for confirmation of pregnancy, for dogs and other animals that have litters, it is not an accurate method of counting.
I want to state that all radiation is harmful. I also want to state that we are all exposed to radiation daily, so while radiation exposure is both harmful and unavoidable, the most important thing is to know specific risks from different types of radiation, and cumulative lifetime exposure. Please see a few paragraphs down for examples com common radiation exposures for perspective on this.
One of the biggest reasons breeders fear x-rays is that as humans any time we go in for an x-ray, there are signs and papers we have to sign to certify that we are not pregnant.
It is very important to understand that x-ray risk during gestation varies depending on how far along a pregnancy is and how much radiation is involved.
Unfortunately, we have no decent canine data I’m aware of. There is, however, a great deal of human data. While dogs are not humans and there are many differences, this is what we currently have, and it’s up to you and your vet to determine if the differences are acceptable to you in your own risk assessment.
When we compare human to canine pregnancy, the timelines are very different. Stages of development, however, are not, so we will need to look at stage of development rather than week of gestation.
One common objection to pre-whelp x-rays is that medical personnel shield themselves when taking x-rays. People involved in care have a much higher exposure than the general public. An x-ray tech can have a dozen or more exposures in a day. This can add up quickly and is a vastly different situation than a single x-ray, or even a few x-rays.
Risk by fetal development
For humans, the risks are much higher during the developmental period from conception through the first 6 weeks. This equates to the developmental periods of pre-implantation and the formation of organs. In dogs, implantation occurs by day 22 post-ovulation and organogenesis is complete by day 35.[2,3] Since x-rays typically aren’t performed until day 50 or later, there is ample time between organogenesis and exposure to indicate a reduction in risk.
[As an aside, the paper that talks about formation of organs (organogenesis) provides fantastic discussion of how puppies develop in utero. It’s a little challenging to get through the terminology, but well worth it. It also has excellent pictures and graphics. The link from University of New South Wales is likewise informative with excellent graphics. Please see footnotes for links.]
Risk from radiation doses
Here are the radiation dose risks from several expert sources (again, this is for humans). [There is more than one unit of measurement for radiation. For consistency, I’m going to use mSV in this post, although there are other measurements for radiation (mrem, rads, etc).]
The US Centers for Disease Control and Prevention (CDC) state that cancer-causing or intellectual effects in human fetuses are not expected at doses under 500 mSV after implantation and formation of organs.
According to the Health Physics Society, the radiation exposure from most diagnostics (<50mSv) for human fetuses does not increase risk of non-cancerous defects or miscarriage during any stage of human fetal development. The reported radiation dose at which we see higher rates of miscarriage and birth defects is above 200 mSv. This paper also states that in later pregnancy, risk of radiation exposure for the fetus is likely comparable to the risk to the mother.
Common radiation doses
The first thing to note is that radiation exposure from digital x-rays is significantly less than the old-school x-rays. Some of us may have the impression of higher exposure if we are older or have lived in areas where technology was slower to become available.
Perspective always helps, so let's look at how the radiation dose from a single digital abdominal x-ray compares to other common exposures. The exposure from a single digital abdominal X-ray is 0.7 mSv.
[These numbers have been assembled from the charts listed below and from the US EPA and sources cited previously in this post.[4,5,6]
Another risk difference between dogs and humans is that humans live a lot longer than dogs, so cumulative exposure increases with lifespan and thus risk for disease from radiation is less for a species that doesn’t live long.
Accuracy of pre-whelp x-rays
An x-ray is only as good as the person reading it. So if you have a vet experienced in reading prenatal x-rays, then you will get good information. Most general vets are not experienced reading pre-whelp x-rays (although some are). You need to make sure that the skill of the practitioner fits your needs, regardless of the test or procedure you are using. Prenatal x-rays can have good accuracy when read by an experienced vet and be useless when read by others. Like anything else, you need to make sure you are using the right professional. If you are in an area where you don’t have access to a skilled vet then you need to determine the value of x-rays and your own skill in reading them.
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 Placentation in Dogs and Cats. Colorado State University. http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/dog_cat.html Last accessed 14 Feb 2020  Godoy, Naira & AF, Souza & JB, Casals & Roballo, Kelly & Ambrosio, Carlos & Martins, Daniele. (2015). Comparative Development of Embryonic Age by Organogenesis in Domestic Dogs and Cats. Reproduction in Domestic Animals. 10.1111/rda.12539. https://www.researchgate.net/publication/277012925_Comparative_Development_of_Embryonic_Age_by_Organogenesis_in_Domestic_Dogs_and_Cats  https://embryology.med.unsw.edu.au/embryology/index.php/Carnegie_Stages Last accessed 14 Feb 2020  Radiation and Pregnancy. A Fact Sheet for Clinicians. CDC, last accessed 14 Feb 2020 https://www.cdc.gov/nceh/radiation/emergencies/prenatalphysician.htm?CDC_AA_refVal=https%3A%2F%2Femergency.cdc.gov%2Fradiation%2Fprenatalphysician.asp  R. Brent. Pregnancy and Radiation Exposure. Health Physics Society. Last accessed 14 Feb 2020. http://hps.org/hpspublications/articles/pregnancyandradiationexposureinfosheet.html  Radiation Sources and Doses. US Environmental Protection Agency. Last accessed 14 Feb 2020. https://www.epa.gov/radiation/radiation-sources-and-doses