
I don't breed dogs BUT if I did, I wouldn't show a bitch in-whelp, in fact I wouldn't show any of my dogs until the litter was on the ground and thriving. I am coming partly from having once bred & shown cats, where showing pregnant queens is not allowed, but also from common sense.
There are quite a few different bugs and viruses in all species (including humans!) where if a female encounters that bug for the first time while pregnant, it can and will cause pregnancy issues and/or deformity in the young. This is why teenage girls are routinely immunised against Rubella, which can cause a great deal of problems in a baby if the mother meets the infection during pregnancy with no immunity. Toxoplasmosis, sometimes caught from infected meat or cat faeces, can do the same if the human mother has not previously acquired immunity. I think Listeriosis falls into that category too.
Add to that, if the bitch did pick up some kind of infection, it would probably have to be treated with by the vet... and who is to know the true knock-on effects of any drugs given? Many instances of so-called safe medication actually being found to be harmful to the developing foetus.