I literally just read that excessive inbreeding can cause that to happen. Cant figure out where so I cant link you so just ask Google. Its not the most common reason though. And more often than not its because the babies are weak. Not because they need protein. Also increase protein halfway through pregnancy with a milk one for dogs. If you have more than one male with the pregnant female that can also cause this to happen. But the mothers being hungryfor protein is a grossly over used assumption. They eat LOW protein diets.
my mom never went to college thank you very much.
Yes they eat low protein diets... but ANY pregnant/ lactating mammals need extra during that time