The invention covers a method and apparatus for assessing the health of an infant by accurately estimating the average growth velocity of the infant. The invention employs an exponential model to estimate growth velocity based on at least two weight measurements from different days of life of an infant. The exponential model provides the accuracy and ease of use that are lacking in current methods applied to growth research. The exponential model has numerous features that make it a desirable and powerful model, i.e., (1) it is extremely accurate; (2) it is simple to use, requiring only weight and day of life at 2 time points; (3) it is robust under various testing conditions; and (4) it is unaffected by clinical factors found commonly among ELBW infants, which allows its broad application for the study of infant growth. The exponential model closely approximates the accurate standard GVs, regardless of starting point or time interval used. An exponential model assumes that growth occurs at a fraction of the previous weight. Although growth does not occur at a constant rate of change, the average GV smoothes the variability in weight gain observed clinically, thereby providing an estimate of a theoretically constant rate of change. The average growth velocity is thus a useful in assessing the health of an infant.