Percentiles and Z-scores are a widely used format to display and interpret growth measurements. In childhood, sex and age-adjusted percentiles and Z-scores for weight, height (supine length if aged less than two years), BMI and ponderal index (at birth) are routinely used in clinical practice and in research studies. These values are used to assess growth and nutritional status in infancy, childhood and adolescence (up to 19 years of age) They are derived by comparing individual growth measurements against growth data or growth charts from a reference (‘normal’) population (a “growth reference” or “growth standard”).
To calculate percentiles and Z-scores, you need:
Percentiles and Z-scores are routinely used in clinical practice to assess and monitor children’s growth and nutritional status and are also widely used in the analysis of data from child nutrition surveys and epidemiological studies. The following indices are expressed as percentiles or Z-scores:
In clinical practice, growth percentiles are commonly derived by manual plotting of the child’s growth measurement against their age on a sex-appropriate paper growth chart to visually indicate their approximate percentile position or band (e.g. “between 85th to 97th”) (see Figure 1).
Alternatively, percentiles can be more precisely converted from Z-scores (see section below).
Figure 1 WHO Child Growth Standards Weight-for-age: Birth to 5 years percentiles chart for girls.
A Z-score is calculated using the formula (assuming a normal distribution):
Measured value – Average value in the reference population
Standard Deviation of the reference population
Average values and standard deviations for various reference populations are available adjusted for age and sex and indicating the appropriate transformation to achieve a normal distribution (see below).
Calculation of Z-scores by hand is laborious. Fortunately, several computer programs are available and data can be entered either one child at a time or using spreadsheets for large numbers of children (e.g. http://www.who.int/childgrowth/software/en/).
It is very important to note the units of measurement required by each of the programs. For example: weight in kg; height/length in cm; age in decimal years, months or days; and sex as M for male and F for female.
Percentiles and Z-scores are easily interchangeable using a table of Z-scores and their associated areas (percentile = the area from infinity to Z) or a calculator.
In Microsoft Excel: To convert Z-scores to percentiles, use the Norm.Dist function with the settings: mean=0; SD=1; cumulative=TRUE.
Which growth reference?
It is important to select the reference population carefully, with consideration of the aim of the assessment or study. For example, older growth reference data might be appropriate if analysing data from an historical study or survey, or to assess secular trends in growth.
Some countries (e.g. UK, USA, and several other European countries) have growth reference data based on their national population (see below). Such growth references are representative of children in those populations. Hence, these percentiles and Z-scores tell you where a measured child lies in relation to that population, but they do not imply that a normal growth value or pattern is healthy.
WHO International Growth Standards and Growth References
The World Health Organisation (WHO) provides international growth standards for children aged 0-5 years and international growth references for children and adolescents aged 5-19 years. While both are intended to be used for children of any nationality, there is an important distinction between standards and references.
Birth weight and birth length reference data are available for preterm infants born from 24 weeks gestation onwards. For example, the INTERGROWTH-21st growth references.
For assessments of postnatal growth, prematurity needs to be taken into account. Age-adjustment is for children born before 37 weeks gestation. For example, if an infant is born 6 weeks early (at 34 weeks gestation), he/she will have a corrected age of 6 weeks less than his/her actual age since birth (compared to 40 weeks of gestation).
Many growth computer softwares will adjust for gestational age. When manually plotting measurements onto a chart, their measurements should be plotted at their actual age, with a horizontal line drawn back to their corrected age. The adjustment should continue to be made until the child is one year old.
The Preterm Postnatal Growth Standards are particularly suitable for monitoring postnatal growth in preterm babies after 32 weeks’ postmenstrual age and may be used for the assessment of preterm infants until 64 weeks’ postmenstrual age (6 months “corrected” age), the time at which they overlap, without the need for any curve adjustment, with the WHO Child Growth Standards for term newborns.
Growth categories based on percentiles and Z-scores
To identify individuals with unhealthy growth: the WHO recommends cut-off values of +/- 2 Z-scores, which roughly correspond to the 2nd and 98th percentiles (Table 1).
Table 1 WHO classification of growth and nutritional status.
|Based on weight and height indices||Height-for-age < -2SD to -3SD||Height-for-age < -2SD to -3SD||Stunted|
|Height-for-age < -3SD||Height-for-age < -3SD||Severely stunted|
|Weight-for-age < -2SD to -3SD||Weight-for-age (up to 10 years) < -2SD to -3SD||Underweight|
|Weight-for-age < -3SD||Weight-for-age (up to 10 years) < -3SD||Severely underweight|
|Weight-for-height < -2SD to -3SD||Wasted|
|Weight-for-height < -3SD||Severely wasted|
|Based on body mass Index (BMI)||BMI-for-age (or weight-for-height) > 1SD||Possible risk of overweight|
|BMI-for-age (or weight-for-height) > 2SD||BMI-for-age > 1 SD (equivalent to BMI 25 kg/m2 at 19 years)||Overweight|
|BMI-for-age (or weight-for-height) > 3SD||
BMI-for-age > 2SD
(equivalent to BMI 30 kg/m2 at 19 y)
|BMI-for-age < -2 to -3 SD||Thin|
|BMI-for-age < -3 SD||Severely thin|
1 Z-score and percentile equivalence: 3SD=0.1%; 2SD=2.3%; 1SD=15.9%; +1SD=84.1%; +2SD=97.7%; +3SD=99.9%.
Adapted from: http://ebook.ecog-obesity.eu/chapter-growth-charts-body-composition/world-health-organization-reference-curves/
To assess and monitor the prevalence of overweight and obesity in populations:
As shown in Figure 2, the (UK) National Child measurement programme (NCMP) and some other national organisations use the BMI-for-age 85th and 95th percentiles cut-offs (equivalent to Z-scores of +1.04 and 1.65, respectively).
Figure 2 Individual and population cut-off values for BMI-for-age percentiles used in the UK.
Considerations relating to the use of percentiles and Z-scores in specific populations are described in Table 2.
Table 2 Application of percentiles and Z-scores in different populations.
|Infancy and lactation||Suitable.|
|Toddlers and young children||Suitable.|
|Adults||Not used – BMI typically implemented|
|Older Adults||Not used – BMI typically implemented|
|Ethnic groups||Suitable in infancy, childhood and in adolescents. Use a population-appropriate growth reference.|
|Other (obesity)||Suitable in infancy, childhood and in adolescents. For extreme values, Z-scores are more informative than percentiles.|
Refer to section: practical considerations for objective anthropometry
Resources are dependent on the instruments/methods used to derive the raw data (weight and height), however computer and freely available software are recommended to derive the scores.
WHO growth reference for school-aged and adolescents.
CDC growth charts from birth to 20 years in the United States web page and PDF.
UK 1990 growth reference: British growth reference, anthropometric data for weight, height, body mass index and head circumference from 17 distinct surveys representative of children in England, Scotland and Wales in 1990.
May 2009 The UK-WHO (2006) growth reference for 0-4 years old children also suitable for moderately preterm infants (32-36 weeks gestation).
May 2009 The UK-WHO growth reference for 2-18 years old children and adolescents.
They intend to assess the growth of school age children and young people in primary or secondary care. The chart includes guidance on the onset and progression of puberty, a BMI centile lookup.
The programmes/macros (LMS macros) for these references can be downloaded here.
The Italian Society for Pediatric Endocrinology and Diabetes (SIEDP)-2002 growth charts for height, weight and body mass index (BMI), to obtain charts (SIEDP-2006) that apply to the Italian population from 2 to 20 years of age, taken as a whole, or separately in two geographical areas (Central-North Italy and South Italy). See here for software and PDF.
Updated in 2014, these include references for height/length, weight and head circumference from 0 to 20 years of age.
Updated in 2002, these include references for height/length, weight and head circumference from 0 to 10 years of age.
Syndrome-specific growth references
Reference charts for syndromes with endocrine features are available for Down Syndrome, Turner syndrome and Achondroplasia. This software also contains over 200 growth charts for different geographical regions. However, charges are applied for its use.