Yeah I know which one you are talking about... Babies and small children need differing levels of body fat to be considered healthy than older children, thus the difference in this chart vs. a traditional bmi calculation.
<a target=_blank class=ftalternatingbarlinklarge href="http://www.cdc.gov/growthcharts/data/set1clinical/cj41l020.pdf">Girls: birth-36 months, weight-for-length</a>
<a target=_blank class=ftalternatingbarlinklarge href="http://www.cdc.gov/growthcharts/data/set1clinical/cj41l019.pdf">Boys: birth-36 months weight-to-length</a>