Growth percentiles have been calculated for American babies by simple statistics: Researchers measured large numbers of children and determined an average. If Jimmy is in the sixtieth percentile in weight, he is heavier than sixty out of every hundred kids. But be careful about reading too much into these percentiles, which have little to do with nutrition and usually reflect simple differences in morphology; they are largely useless in predicting future size.
It’s slightly more useful to compare weight and height percentiles in order to detect overweight and underweight children or at least early warning signs. For example, a child who is in the thirtieth percentile for height and seventieth percentile for weight might be overweight. But numbers are only numbers; when in doubt, look at the kid himself.
Charts and percentiles are also useful in monitoring head growth. Charting the head circumference can flag those rare conditions where head size is abnormally large, which may indicate excess fluid in the brain [See: Head Shape].
Growth charts may look scientific, but they are far from precise. In general, their flaw is that they assume steady growth: According to these charts, skinny babies stay thin through later life. Mother Nature doesn’t always work that way; skinny babies can plump up, and plump babies can thin out.