They don't make it that far. They get eaten long before they're weaned, fyi.This can happen for many reasons, including the mom mouse has dried up & is too old to keep breeding, or bad genetics from the start. In my colony, I know it's not about the food. If it's not the mom's first litter, I would feed her off as a non-breeder. If I notice any cannibalism of a litter, I pull the rest out before they meet the same fate. Sometimes I can sneak them into another mouse's litter (if they're the same size & not too many for her to feed overall), but usually they get fed off or frozen for future feedings. Breeding mice requires some "tough love", lol.