Really only the breeder who is intimately familiar with her lines can tell you whether a puppy's birth weight will be indicative of it's adult weight. More often than not, it is, although there are the situations where the "runt" or the smaller ones of a litter will "catch up". I have four yorkies under 4 lbs and they were all small at birth.
A breeder who has been breeding for many years and knows her lines and which dogs "throw" smaller or larger puppies can usually tell with a great deal of certainty what her puppies will weigh as adults. There is definitely a pateren that develops from certain dogs. |