While NO BREEDER CAN GUARANTEE ADULT WEIGHT, the Chihuahua Weight Chart is great to use as a guideline to estimate a mature weight (within a pound or so). This chart is used only as a guide, as many things can affect the mature weight of a puppy. Weight may vary by a little or a lot, depending on its bloodline, genetics, how well the animal is fed, etc. Weight estimates before the age of 8 weeks old are often inaccurate because the puppy is still nursing or being weaned and may be heavier due to fat from the mother's rich milk. This weight chart only works with purebred Chihuahuas. The weight chart is from a book called "The Complete Chihuahua Encyclopedia" by Hilary Harmar, Pub. 1972, Arco Publishing Co., Inc.