Principles of WHO chart construction
WHO growth charts (nomograms) are specialized coordinate grids designed to compare a child's anthropometric data with their chronological age. This tool is divided into two age categories: from birth to complete 5 years of age (infancy and early childhood) and from 5 to 19 years of age (childhood, puberty, and the completion of linear growth).
A crucial rule is strict gender specification. Standards for boys and girls are presented on separate graphs because the rates of linear growth, absolute values, and timing of puberty differ statistically between the sexes.
How the coordinate grid is structured:
- Vertical axis (ordinate): records the absolute anthropometric indicator in centimeters. For example, for infants under 5 years old, the scale is marked from 45 to 125 cm in 1 cm increments. For the adolescent boy group, the range is from 90 to 200 cm, and for girls, from 90 to 180 cm.
- Horizontal axis (abscissa): displays exact age. The scale is highly detailed and marked in full years with intermediate divisions. For children under 5, the grid step is two months (2, 4, 6, 8, 10), and for the older age group, three months (3, 6, 9). This obliges the physician to calculate the patient's age as accurately as possible at the time of examination.
Standard Deviation Score (SDS) system and centiles
The assessment of physical development is based not on rigid isolated standards, but on the concept of statistical distribution corridors. The curves on the chart reflect the number of standard deviations (Standard Deviation Score, SDS, or Z-score) from the population average.
Decoding the curve values from top to bottom:
- 0 (green line): the median. This is the absolute average, statistically ideal indicator for a specific sex and age (corresponding to the 50th centile).
- Corridor from -1 to +1 SDS: the zone of absolutely normal, or average, growth.
- Corridors from -1 to -2 and from +1 to +2 SDS: indicators below and above average. These are considered variants of normal distribution, but require dynamic monitoring by a pediatrician.
- -2 and +2 lines (red): the boundaries of normal distribution (equivalent to the 3rd and 97th centiles, respectively).
- Zone below -2 SDS: indicates short stature (growth retardation). The patient requires the exclusion of somatic, genetic, or endocrine pathology.
- Zone below -3 SDS (black line): a marker of pronounced deviations, indicating severe growth retardation (subnanism and nanism).
- Zones above +2 and +3 SDS: reflect tall stature or gigantism, which also serves as an indication for an in-depth medical examination.
Measurement rules and the "step" phenomenon
According to WHO recommendations, the methodology for obtaining anthropometric data is strictly tied to the child's age, as body position directly affects the final result.
- Under 2 years old: measurement is performed exclusively in the supine position. The physician evaluates recumbent length.
- Over 2 years old: measurement is performed in the standing position. Height is evaluated.
An interesting methodological feature of WHO nomograms is the characteristic gap in all curves at exactly the "2 years" (24 months) mark. Visually, this looks like a small step with a downward shift in graph values.
This adjustment is intentionally built into the mathematical model. Due to the effect of gravity in the upright position, physiological compression of the intervertebral discs occurs. As a result, a child's standing height will always be approximately 1 centimeter less than their recumbent length measured in the horizontal position.
Growth dynamics and sex differences in puberty
The shape of the percentile curves allows for a clear visualization of growth spurts (pubertal growth spurts) and the timing of the closure of growth plates. In adolescence, sexual dimorphism manifests most vividly.
In girls the curve begins to flatten and transition into a horizontal plateau significantly earlier—at approximately 15–16 years of age. This is due to the earlier onset of puberty. Estrogens produced have a stimulating effect on the closure of epiphyseal growth plates. The final height median for females is set at around 163 cm.
In boys the period of active linear growth lasts longer. The upward trend of the curve persists up to 17–18 years, which is associated with a later onset of puberty. The final height median for males reaches approximately 175–176 cm.
To remember the SDS normal ranges, use the "traffic light rule": Green (0) — ideal median; Yellow (1 to 2) — variant of normal, but requires dynamic attention; Red (beyond ±2) — stop signal, outside the boundaries of normal distribution, examination is needed.
Class and exam questions
Why are the WHO charts separated by sex?
The rates of linear growth, timing of the pubertal growth spurt, and final anthropometric parameters in boys and girls are statistically different, which is why separate growth charts have been developed for them.
What does the step on the WHO chart at exactly 2 years mean?
This gap reflects the transition from measuring length (recumbent) to measuring height (standing). Standing height is always about 1 cm less due to gravitational compression of the intervertebral discs.
From which SDS value is growth retardation diagnosed?
Growth retardation is stated when the indicator drops below -2 SDS from the median, which corresponds to crossing the lower limit of the statistical norm (below the 3rd centile).
Why do girls stop growing earlier than boys?
Early puberty in girls is accompanied by an estrogen surge that accelerates the closure of epiphyseal growth plates. Their plateau occurs at 15–16 years, whereas in boys active growth continues until 17–18 years.
What else to study on this topic
- Exclusion of endocrine pathology in growth retardation (GH deficiency, hypothyroidism)
- Genetic syndromes accompanied by short stature and nanism
- Somatic causes of physical development delay in early childhood
- Etiology and diagnosis of tall stature and gigantism in children and adolescents
- Assessment of body mass index (BMI) and developmental harmony according to WHO criteria
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