Someone asked me yesterday "Why is it ok to radiograph a pregnant dog but not a human?". Coming from a veterinary background, I knew that post 54 days of pregnancy it was safe to radiograph a pregnant dog or cat. I knew that this was because of the state of development, but could not state specific facts.
So, this morning I looked into it.
Irradiation in utero can be detrimental to a fetus -- this we know through historical record. What is a game changer in dogs and cats is the rate of growth, as well as the advancement of radiography itself.
Irradiation is lethal to fetuses close to before or immediately after implantation (Diagnostic Imagining Systems, 2017), can cause malformations during the period of organ genesis, and generally during the later stages of development the risk decreases as acceptable radiation dose approaches adult values (I.e. the older a fetus is, the better able to withstand irradiation).
Radiography itself has advanced greatly in even just my lifetime. When I started in vet clinics exposure times were less specific, radiation values were more sporadic, and radiograph development involved direct contact with heavy metal chemicals. Since then, with the advent of digital radiography, a lot of the danger has been removed. Exposure times are calculated and programmed into the machines, taking out guess work, and there is no development! Instead the plate is read by a digital processor and the image transferred directly to a computer!
Want to know how long a bitch is exposed to irradiation for her rad? 0.12 seconds. Also, compared to human values, the amount of radiation used is miniscule (MA = 4 vs MA = 20).
So, given that the pups are days away from birth, the risk to them via irradiation is minimal. The safety factor gained from having a fairly certain idea of how many puppies to expect, as well as an idea of their positions, is in my opinion, worth it.