Assessment of body weight and BMI in children

Weighing rules and calculation algorithms

Body weight helps screen for conditions associated with undernutrition or overnutrition. To ensure data reliability, weighing is performed using specialized equipment with an accuracy of up to 0.1 kg.

Body position depends on the patient's age:

• Infants (under 1 year): positioned on the scales sitting or lying down.
• Children over one year: weighed exclusively standing.

The primary criterion for establishing a diagnosis is the body mass index (BMI). The main rule of pediatrics is that the absolute value of the index is never analyzed in isolation. The obtained figure must be compared with World Health Organization standards (2007 standards), taking into account gender and exact age.

Principle of deviation assessment (SDS system)

To interpret BMI, specialized WHO growth charts are used, operating on the basis of standard deviations — Standard Deviation Score (SDS, or Z-scores).

The central green line of the chart is the median (0), showing the ideal average statistical indicator for a specific age and gender.

All deviations from the median are divided into:

1. Positive (above zero): +1, +2, +3 SDS lines. They indicate excessive weight accumulation and various stages of obesity.
2. Negative (below zero): -1, -2, -3 SDS lines. They indicate nutritional deficiency and the development of protein-energy malnutrition.

Features of growth charts from birth to 5 years

Charts are strictly separated by gender (nomograms for boys are usually located at the top, for girls at the bottom). The vertical BMI scale ranges from 10 to 22 kg/m². The horizontal age axis up to one year is marked with a 2-month step, and after one year — monthly.

The physiological dynamics of the curve have striking features:

• Growth period: from birth to 6–8 months, the index grows rapidly as the infant actively accumulates adipose tissue.
• Decrease period: after 8 months, a smooth decline begins, continuing up to 5 years. The child stretches in height, and body proportions change.
• Methodological "step": exactly at the "2 years" mark, a characteristic gap is visible on all curves. This is due to the fact that up to 2 years, body length is measured (recumbent), and after — height (standing). The difference in methods results in a minor change in the final calculation.

Indicator dynamics in children from 5 to 19 years

At an older age, separate nomograms are used (BMI axis from 12 to 36 kg/m²). The age scale is divided into years with a step of one quarter (3, 6, 9 months).

During this period, the curves become non-linear: the physiological BMI naturally increases as children grow older. A graphical widening of the corridors is also noticeable (the distance between the extreme values of +3 and -3 SDS increases). This reflects the high variability of norm and pathology in pubertal and prepubertal periods, which is directly related to hormonal changes.

Diagnostic criteria for threshold values

Interpretation of results is carried out by comparing SDS lines and percentile distributions:

• Obesity: SDS indicator exceeds +2 (corresponds to >97th percentile). At 19 years old, this is equivalent to an adult BMI of 30 kg/m².
• Overweight: indicator is above +1 SDS (>85th percentile). Requires pediatrician's attention. Equivalent at 19 years is 25 kg/m².
• Wasting (thinness): indicator drops below -2 SDS (below 3rd percentile), which indicates undernutrition.
• Severe thinness: drop below -3 SDS (below 1st percentile), characteristic of severe protein-energy malnutrition.

Class and exam questions

What else to study on this topic

• Features of adipose tissue accumulation in the first year of life
• Pathogenesis of protein-energy malnutrition
• Gender differences in body composition changes during puberty
• Clinical assessment of percentile corridors

Other topics in "Chapter 1. General Issues"

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