Growth chart glossary

CDC 2000 Growth Charts

The reference published by the US Centers for Disease Control and Prevention in 2000, revising the 1977 NCHS charts, and built from national survey data on American children. It is a descriptive reference: it reports how the surveyed population actually grew. US practice uses it from 2 through 19 years.

Growth chart

A published set of curves showing how a measurement such as weight, length or head circumference is distributed across a reference population at each age. The curves are drawn from fitted parameters rather than recounted children, and the same child can carry slightly different percentiles on two different charts.

Head circumference

The largest circumference of the head, formally the occipitofrontal circumference, measured with a non-stretch tape passing just above the eyebrows and ears at the front and around the most prominent part of the skull at the back. The largest reading found is the one the chart wants.

LMS parameters

The three numbers each agency publishes at every age point to define a growth curve: L, a Box-Cox power that accounts for the skew of the distribution, M, the median, and S, the coefficient of variation. Every percentile on a growth chart is computed from these three numbers, which is why the published tables are only three columns wide.

Percentile

A rank position inside a reference population. A measurement on the 30th percentile means about 30 children in 100 of the same age and sex in the reference group measure less, and about 70 measure more. It describes position, never health, and there is no percentile a child is supposed to be on.

Recumbent length

A child's crown-to-heel measurement taken lying down, on a length board with a fixed headpiece and a sliding footpiece. It is the length measurement used under 24 months, and it reads about a centimetre longer than the same child measured standing, because the spine compresses under load.

Reference population

The specific group of children a growth chart was built from. Every percentile is a position inside that group and nothing else, which is why the chart a number came from is part of the number's meaning.

Stature

A child's standing height, measured heels to crown against a flat vertical surface. It replaces recumbent length from 24 months, which is the same age at which the reference table switches from the WHO standard to the CDC charts.

WHO Child Growth Standards

The reference published by the World Health Organization in 2006, built from the WHO Multicentre Growth Reference Study which followed roughly 8,500 children across six sites in Brazil, Ghana, India, Norway, Oman and the United States between 1997 and 2003. It is a prescriptive standard: it describes how children grow under conditions believed to support healthy growth, with every infant in the reference group breastfed. US practice uses it from birth through 24 months.

z-score

How many standard deviations a measurement sits from the median of the reference population, after the skew of the distribution has been accounted for. A z of 0 is the 50th percentile, plus 1 is about the 84th, and minus 2 is about the 2nd. Unlike a percentile, a z-score is evenly spaced, so a change in z means the same thing in the middle of the chart as in the tails.

The maths these terms describe is worked through in what a percentile actually means, and the published values themselves are on the reference tables page.