I think that it's pretty common for a dead baby or two to happen here and there. Most critters who give birth to litters instead of singletons lose a few occasionally. When I bred dogs it wasn't unusual in a litter of 11 or 12 (hounds have huge litters) to have one or two dead pups. Often they were very small or deformed and my vet said that this can happen because they are on the farthest end of the uterine horn and don't receive much blood supply and that when they make it they are the runts. If the female had a lot of dead ones every litter that would be another story.

One of my rats gave birth to 7 for her first litter. I wait and see what the second and third litters are like before I start thinking about culling them from breeding. Incidentally, her 7 were born about twice the size of the 12 that my other rat just dropped.

I suspect that we don't see a lot of the dead rodent babies that do occur because the mother's instinct is to eat afterbirth and dead babies--if she didn't they could bring disease to the others as they decompose and her instincts tell her that the smell would draw predators.