Mother dog reject their new puppies for many different reasons. Although many people believe mother dogs will always reject the runts of their litters, this is not always the case.
Problems Facing Runts
Typically, runts are the babies who were positioned in the middle of the mother’s uterus, where they received less access to nutrition. Low birth weight can be a risk factor for health problems, including fading puppy syndrome, which often results in death. Runts who survive their first 6 to 8 weeks are likely to make it to adulthood.
Mothers & Runts
Contrary to popular belief, mothers usually do not reject the runt of the litter. In fact, mother dogs often try to protect them from their competitive siblings. Because they're smaller, runts tend to have a harder time competing with siblings for warmth and food. When mothers do reject the runt, it's not because he's simply too small. Mothers know when a puppy is too weak to survive, and they'll refrain from giving their resources to the pup. Except for those born with congenital malformation, most abandoned runts can be saved by hand-rearing away from the litter.