Growth standards, Z-scores and growth charts
A child's measurement is compared with the WHO Child Growth Standards (2006). Z-scores and growth charts show whether the child is growing along a healthy curve.
Growth standards and indices
Growth is judged by comparing a child's weight and height with a reference. India now uses the WHO Child Growth Standards (2006), built from healthy, breastfed children in six countries, India among them. They show how children should grow when well fed and cared for. Older texts used the NCHS reference and per cent of the median (Gomez, IAP). The three main indices are weight-for-age (underweight), height-for-age (stunting) and weight-for-height (wasting); BMI-for-age is used for older children and adolescents.
Worked example: Z-score and per cent of median
A boy aged 24 months weighs 9.4 kg. Take the reference median weight for his age as 12.2 kg and the reference SD (below the median) as 1.25 kg. Find his weight-for-age Z-score and his weight as a per cent of the median, and classify him. (Values are close to the WHO table and rounded for practice.)
- Z-score = (observed − median) ÷ SD.
- = (9.4 − 12.2) ÷ 1.25 = −2.8 ÷ 1.25 = −2.24.
- −2.24 lies between −2 and −3, so he is moderately underweight.
- Per cent of median = 9.4 ÷ 12.2 × 100 = 77.049…, rounded to one decimal = 77.0%.
- By Gomez (75-90%) this is grade I; by IAP (71-80%) it is also grade I, i.e. mild.
Z-score −2.24: moderately underweight. The per-cent systems call him only mildly undernourished, which is one reason WHO prefers Z-scores: they are more exact at the low end and comparable across ages and indices.
Classification systems at a glance
| System | Based on | Grades |
|---|---|---|
| WHO Z-scores (current) | Each index against the WHO 2006 standards | Normal −2 to +2 SD; moderate below −2 to −3 SD; severe below −3 SD |
| Gomez | Weight-for-age, % of median | Normal above 90%; grade I 75-90%; grade II 60-75%; grade III below 60% |
| IAP (Indian Academy of Paediatrics) | Weight-for-age, % of median | Normal above 80%; grade I 71-80%; II 61-70%; III 51-60%; IV 50% or less |
| Waterlow | Weight-for-height (wasting) and height-for-age (stunting), % of median | Wasting: normal above 90%, severe below 70%. Stunting: normal above 95%, severe below 85% |
| Wellcome | Weight-for-age % with or without oedema | Underweight, kwashiorkor, marasmus, marasmic kwashiorkor |
Activity: plotting and interpreting a growth chart (under five)
- 1Choose the right chart: the WHO weight-for-age chart for a boy or a girl, as printed on the Mother and Child Protection (MCP) card.
- 2Work out the child's age in completed months from the date of birth.
- 3Weigh the child accurately, to 0.1 kg.
- 4★Find the age on the horizontal axis and the weight on the vertical axis; put a dot where they meet.
- 5Join this dot to last month's dot to make the growth line.
- 6Read the band: above the −2 SD curve is normal; between −2 and −3 SD is moderately underweight; below −3 SD is severely underweight.
- 7★Read the direction of the line (see the table below) and act: praise, counsel, follow up or refer.
★ one-way, controls the pace
Reading the growth line
| Direction of the line | Meaning | Action |
|---|---|---|
| Rising, parallel to the curves | Healthy growth | Praise the mother; continue feeding advice |
| Flat (no weight gain) | Growth faltering: a warning | Check feeding and illness; follow up next month |
| Falling | Losing weight: danger | Urgent check-up and extra feeding; refer if ill |
| Below the −3 SD curve | Severely underweight | Check for SAM (weight-for-height, MUAC); refer to an NRC if there are complications |
On the MCP card, the mother herself can see whether her child's line is rising, flat or falling. Explaining the line at every weighing turns growth monitoring into growth promotion.
2 marks: “What is a Z-score?” The number of standard deviations a child's measurement lies above or below the reference median: (observed − median) ÷ SD. Below −2 is moderate and below −3 severe undernutrition.
5 marks: “How will you plot and interpret a growth chart for a child under five?” Chart by sex; age in completed months; accurate weight; plot the dot; join to the last dot; read the band and the direction of the line; counsel, follow up or refer.
10 marks: “Explain growth monitoring and the classification of undernutrition in children.” WHO 2006 standards and indices; Z-score formula with a worked example; Gomez, IAP, Waterlow and Wellcome; plotting and reading a growth chart; growth promotion.
Below −2 = moderate, below −3 = severe. On the chart, read the direction of the line, not just the dot.