Are infant and toddler size-weight percentiles useful things for parents to be looking at, as opposed to just aggregations of data for physicians to use?
The latter. Dear God, the latter.
Weight, height and head circumference percentiles are similar to developmental milestones in their utility. They are useful for medical providers to track growth over time, and to detect abnormalities. They are not useful as snapshots, and they are not worthwhile as a point of parental fixation.
Sometimes they are helpful visual aids when I am trying to help parents grasp the significance of a problem they may be denying. If a child’s weight is soaring off the top of the chart or plunging through the bottom, showing the parents the curve can facilitate communication. But in almost all cases, the problem is visible to the naked eye anyhow. If the kid looks basically proportional and well-nourished, then chances are very good that his height and weight are fine.
The same is true of head circumference. If your child’s head looks normal as compared to the rest of her, it almost certainly is normal. Where it falls at any given time on the growth curve is probably not important. The growth chart can be useful if the head is growing much faster or slower than height and weight, but it’s not something I would encourage parents to worry about.
But parents always want to know their child’s percentages, and often evince anxiety when their child is a little bit higher or lower on the curve than seems normal. In these situations I advise them to believe their eyes, and be reassured by their child’s healthy appearance. I tell them (as is almost always true in almost all situations) that if their child seems fine, he or she probably is. If it’s a chart vs. their own sense (and mine), their sense is almost certainly right. Growth charts are helpful in detecting those rare occasions when things aren’t all right, but it’s my job to pay attention to these things, not parents’.