Growth charts, a norm or a description

A reference chart describes growth as observed in a given population, as it is. A standard chart describes growth observed under conditions judged favourable, and therefore works as a norm.

The World Health Organization published in 2006 charts of the second type, built from a multi-country sample of children raised under conditions defined by the protocol. Earlier charts described populations as they were. Those conditions included in particular the absence of maternal smoking and breastfeeding in line with the organisation's recommendations.

French health records long used charts derived from a French population of the 1950s and 1970s. New charts, built on recent French data, were introduced in 2018. They were drawn up from measurements collected during routine consultations, which provided a very large number of observations.

The choice not to adopt the WHO charts in full was debated. Its defenders invoke fit with the population being followed, its critics the value of a common international norm. The debate does not set a good chart against a bad one, but two uses: describing the population being followed, or relating it to a common norm.

A percentile situates a child relative to a distribution, not relative to an ideal. By construction, a fixed share of children falls below the third percentile, whatever the health of the population. Parents' height is moreover among the elements taken into account when interpreting a position, which shows that a percentile is never read in isolation.

Clinical reading therefore bears less on position than on trajectory: a change of channel, that is a departure from the curve followed until then, is the information monitoring seeks to produce.

Moving from one reference to another mechanically changes the number of children classified below a threshold, without any child having changed. This has been documented during chart transitions in several countries. The phenomenon is arithmetical: if the new distribution is shifted, the same child changes percentile without having grown taller or heavier in any different way.

The consequences are concrete: referral for further examination, prescription of follow-up, estimation of the prevalence of overweight or underweight in public statistics.

In children, this index varies sharply with age, which requires specific charts and forbids the use of adult thresholds. It decreases during early childhood before rising again, and the timing of that rise, known as adiposity rebound, has itself been the subject of much research.

It does not distinguish body composition, a known limit restated in reference documents, which is why it is presented as a population screening tool rather than an individual measure.