Well before my doctoral dissertation studies percent weight change was used in developing areas of the world to determine which families were eligible for nutritional supplements for their infants and young children and/or eligible for nutritional counseling.  Percent weight loss was abandoned because it does not nearly as reliable as tracking growth along a percentile (or z-score curve).  The reason why this is so is because as infants and young children get older, their original weight increases so that a particular percent of weight loss may not coincide with the percentiles of normal growth.  I remember calculating sensitivity and specificity and positive and negative predictive values comparing cutoffs using percent weight loss and percentile curves and they do not match.  This was back in the late 1980s. 

So, for older babies, percent of weight change is NOT an indicator that should be used.  The WHO growth charts SHOULD be used and there are criteria on their website for how to interpret drops in percentILES.  

The only use I know for knowing percent weight loss is that first check in the first week of life that can never really be used without ALSO considering other indicators as well.  The simplest way to calculate percent weight loss in backward countries like the United States that use pounds and ounces (which causes enough math errors to hit in my top 10 list of reasons why supplementation is unnecessarily used and cases of failure to thrive are missed) is to simply switch the scale from pounds and ounces to grams.  All good scales have that switch.  Most babies are weighed in grams at birth.  Simply subtract the current weight in grams from the birth weight in grams.  Move the decimal point one digit to the left for the birth weight.  You can quickly see if the weight loss is greater than 10% in grams.    

Susan E Burger, MHS, PhD, IBCLC, RLC

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