A mоther is cоncerned thаt her 7-mоnth-old breаst fed infаnt is not getting enough to eat. The infant weighed 7 lb, 8 oz at birth and was 19 inches long. Today (at 6 months of age) the infant weighs 15 lb and is 25 inches long. The family nurse practitioner's response is based on the knowledge that: