Prevalence of Underweight

Impact

% of children aged 6–59 months with a weight-for-age z-score < −2

The indicator measures the prevalence of underweight among children aged 6–59 months in the assessed population. Underweight means low weight for age relative to the WHO Child Growth Standards. A low weight-for-age z-score can reflect stunting, wasting, or both, so underweight is a broad indicator of undernutrition and does not distinguish between acute and chronic growth deficits. Where possible, interpret it alongside more specific anthropometric indicators such as height-for-age and weight-for-height.

Assess the age, sex and weight of a representative sample of children aged 6–59 months from the population for which underweight prevalence is to be estimated. Measure weight using standardised anthropometric procedures and appropriately calibrated equipment. SMART methodology can be used, particularly where a SMART nutrition survey is appropriate, but it is not the only acceptable survey approach.

Calculate weight-for-age z-scores using the WHO Child Growth Standards. The numerator is the number of surveyed children with a valid weight-for-age z-score < −2 SD; the denominator is all surveyed children aged 6–59 months with a valid weight-for-age z-score. Multiply the result by 100 to obtain the percentage.

For programme baseline and endline comparisons, use the same indicator definition, age range, target population and comparable sampling and measurement procedures. Determine sample size according to the survey objective, expected prevalence, sampling design and required precision.

Report prevalence estimates with confidence intervals and use an appropriate statistical comparison when assessing differences between survey rounds. Review anthropometric data quality and apply appropriate validity and plausibility checks before interpreting the results.

This data can be disaggregated by sex, age group and geographic area. Interpret subgroup results only where the sample size provides adequate precision.

1) Accurate age assessment is essential because the weight-for-age z-score depends directly on the child’s age. Use reliable documentation such as birth or health records where available and apply the age-assessment procedures specified in the survey protocol. Where the exact date of birth is unknown, locally adapted events calendars and appropriate caregiver probing can support age estimation (see FAO guidance below).

   

2) Underweight prevalence can be influenced by changes in the wasting component of weight-for-age. Where nutritional status varies seasonally, collect repeat surveys at comparable times of year and consider changes in food security, illness and other contextual factors when interpreting trends.

3) Underweight is less specific than stunting or wasting. A child with low weight-for-age may be stunted, wasted, or both, so this indicator cannot determine which form of undernutrition is responsible. Do not use underweight prevalence alone to estimate the prevalence of acute malnutrition or to infer its causes. Where feasible, interpret WAZ together with height-for-age and weight-for-height and/or other relevant nutrition indicators.

  

4) The standard WHO global under-five indicator covers children aged 0–59 months, while this IndiKit indicator uses 6–59 months, consistent with the age range commonly used in SMART surveys. When comparing the result with national, UNICEF/WHO/World Bank or other external estimates, check that the age ranges are the same; estimates based on different age ranges should not be treated as directly comparable.

5) Sample size and precision should be determined from the survey objective, expected prevalence, sampling design and available resources rather than by applying a fixed margin of error. SMART states that there is no single standard precision for all surveys. When comparing survey rounds, plan the sample size around the size of change that needs to be detected and interpret changes using confidence intervals or an appropriate statistical comparison.

Also consider using the following indicators:

• Prevalence of Stunted Children (distinguishes impaired linear growth, which underweight alone cannot identify)

• Prevalence of Global Acute Malnutrition Based on WHZ (provides a more specific measure of wasting and acute nutritional status)

• Minimum Acceptable Diet (measures a proximal child feeding outcome that many nutrition programmes can influence more directly)

• Prevalence of Diarrhoea Among Children (captures an important illness pathway that can affect child weight and nutritional status)