It's okay to get the runt of the litter if the puppy is healthy after the 8-week period with its mother. The runt can range from being the smallest within a perfectly healthy weight range to having a low birth weight. If the puppy is a normal weight for their breed size, it will be just as healthy as its littermates. If the runt of the litter has a low birth weight, it may need some help along the way to keep it healthy. However, if the runt lives for 10 days, it's usually in the clear.