Per the UCDavis Veterinary School website, the following is cited:
Radiographs
Twenty-eight days into pregnancy, the bones of the fetus begin to become calcified. At 42-45 days of pregnancy, radiographs can be taken to visualize the pups in-utero. However, for best results, radiographs should be taken more than 47 days post breeding, when fetal skeletons are more readily visible.
Radiographs can help determine the number of fetuses, though it is less accurate in bitches carrying large litters. Fetal viability cannot be determined by radiograph unless advanced lesions exist in the fetus, such as skeletal collapse or gas within the fetus.
We do ours around day 52 or so. Good luck with your pups.
