A growth percentile is the proportion of children in a reference population who were smaller than a given child at the same age and sex. A length at the 40th percentile at 6 months means that 40% of infants in the reference population were shorter and 60% were longer.

The percentile is a position within a distribution. The 50th percentile is the median of the reference population, not a target. In a healthy population, one tenth of children are below the 10th percentile by definition.

Two reference charts

In the United States, two references are used depending on age, and the growth planner follows the same convention.

From birth to age 2, the reference is the World Health Organization's 2006 standard. It was developed from about 8,500 children in six countries who were breastfed, born to non-smoking mothers, and raised in conditions without major constraints on growth. The WHO chart describes growth under favorable conditions and is intended as a standard.

From age 2, the reference is the CDC 2000 chart, developed from U.S. national survey data collected between 1963 and 1994. It describes how American children grew during that period without selection for feeding or environment. The Centers for Disease Control and Prevention recommended this combination in 2010. At the transition between charts, a child's percentile can change by a few points with no change in the child, because the two references describe different populations.

How the curves are constructed

Each age on a chart is described by three parameters: L, M and S. M is the median, S is the coefficient of variation, and L describes the skewness of the distribution. Cole and Green published the method in 1992, and both the WHO and CDC charts use it.

To position a child, the calculator converts the measurement to a z-score using those three parameters, then converts the z-score to a percentile. A z-score of 0 corresponds to the 50th percentile, +1 to about the 84th, and -1 to about the 16th. Z-scores are evenly spaced. A change from the 3rd to the 10th percentile is the same distance as a change from the 50th to the 75th, although the percentile figures suggest otherwise.

The planner uses the same calculation in reverse to project growth. It holds the child's current z-score constant, reads the three parameters at each future age, and calculates the measurement corresponding to that z-score. The result page refers to this as the current growth channel.

Percentile crossing in infancy

Crossing percentile lines in the first two years is common among healthy children. Mei and colleagues (2004) examined this in a large U.S. sample. Shifts across two major percentile lines were frequent, most often in the first six months. They became uncommon after age 2. Size at birth reflects conditions during pregnancy as well as the child's own growth pattern. Infants born large or small tend to move toward the median as their own pattern becomes established.

Measurement error also contributes. Infant length is measured lying down, and an infant who is not fully extended can measure a centimeter shorter. Weight varies with feeding and diaper state. A percentile from a single visit has a margin of error. Two visits a few weeks apart can fall on either side of a line without any change in growth.

When a change is significant

Clinicians look for a sustained change in one direction across several visits, particularly a downward change, particularly after infancy, and particularly when weight and length are moving differently. A child who has tracked the 25th percentile for three years and then falls to the 5th over six months shows one pattern. An infant who moves from the 90th to the 60th percentile by the first birthday shows a different one.

The planner does not assess growth in this way. It takes the current position as given and projects it forward to answer one question. If the child continues along the current channel, when will they reach a given clothing or car-seat limit? The planner's ranges are scenario bounds produced by repeating the projection one channel higher and one channel lower. They do not estimate the probability that the child remains in the current channel.

Interpreting the number

A percentile describes where a child is positioned today relative to the reference population. A change in percentile is a question for the next visit rather than a conclusion. The 50th percentile is the center of the distribution, and half of all children are on either side of it.