Percentiles and Zscores are a widely used format to display and interpret growth measurements. In childhood, sex and ageadjusted percentiles and Zscores 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 Zscores, you need:
Percentiles and Zscores 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 Zscores:
Percentiles
In clinical practice, growth percentiles are commonly derived by manual plotting of the child’s growth measurement against their age on a sexappropriate paper growth chart to visually indicate their approximate percentile position or band (e.g. “between 85^{th} to 97^{th}”) (see Figure 1).
Alternatively, percentiles can be more precisely converted from Zscores (see section below).
Figure 1 WHO Child Growth Standards Weightforage: Birth to 5 years percentiles chart for girls.
Source: http://www.who.int/childgrowth/standards/chts_wfa_girls_p/en/
Zscores
A Zscore 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 Zscores 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 Zscores are easily interchangeable using a table of Zscores and their associated areas (percentile = the area from infinity to Z) or a calculator.
In Microsoft Excel: To convert Zscores 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 Zscores 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 05 years and international growth references for children and adolescents aged 519 years. While both are intended to be used for children of any nationality, there is an important distinction between standards and references.
Preterm infants
Birth weight and birth length reference data are available for preterm infants born from 24 weeks gestation onwards. For example, the INTERGROWTH21^{st} growth references.
For assessments of postnatal growth, prematurity needs to be taken into account. Ageadjustment 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 Zscores
To identify individuals with unhealthy growth: the WHO recommends cutoff values of +/ 2 Zscores, which roughly correspond to the 2^{nd} and 98^{th} percentiles (Table 1).
Table 1 WHO classification of growth and nutritional status.

cutoff points 
cutoff points 


Based on weight and height indices  Heightforage < 2SD to 3SD  Heightforage < 2SD to 3SD  Stunted 
Heightforage < 3SD  Heightforage < 3SD  Severely stunted  
Weightforage < 2SD to 3SD  Weightforage (up to 10 years) < 2SD to 3SD  Underweight  
Weightforage < 3SD  Weightforage (up to 10 years) < 3SD  Severely underweight  
Weightforheight < 2SD to 3SD  Wasted  
Weightforheight < 3SD  Severely wasted  
Based on body mass Index (BMI)  BMIforage (or weightforheight) > 1SD  Possible risk of overweight  
BMIforage (or weightforheight) > 2SD  BMIforage > 1 SD (equivalent to BMI 25 kg/m^{2} at 19 years)  Overweight  
BMIforage (or weightforheight) > 3SD 
BMIforage > 2SD (equivalent to BMI 30 kg/m^{2} at 19 y) 
Obese  
BMIforage < 2 to 3 SD  Thin  
BMIforage < 3 SD  Severely thin 
^{1 }Zscore 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.ecogobesity.eu/chaptergrowthchartsbodycomposition/worldhealthorganizationreferencecurves/
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 BMIforage 85^{th} and 95^{th} percentiles cutoffs (equivalent to Zscores of +1.04 and 1.65, respectively).
Figure 2 Individual and population cutoff values for BMIforage percentiles used in the UK.
Strengths
Limitations
Considerations relating to the use of percentiles and Zscores in specific populations are described in Table 2.
Table 2 Application of percentiles and Zscores in different populations.
Population  Comment 

Pregnancy  Not used. 
Infancy and lactation  Suitable. 
Toddlers and young children  Suitable. 
Adolescents  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 populationappropriate growth reference. 
Other (obesity)  Suitable in infancy, childhood and in adolescents. For extreme values, Zscores 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
WHO growth reference for schoolaged and adolescents.
USA
CDC growth charts from birth to 20 years in the United States web page and PDF.
UK
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 UKWHO (2006) growth reference for 04 years old children also suitable for moderately preterm infants (3236 weeks gestation).
May 2009 The UKWHO growth reference for 218 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.
Italy
The Italian Society for Pediatric Endocrinology and Diabetes (SIEDP)2002 growth charts for height, weight and body mass index (BMI), to obtain charts (SIEDP2006) that apply to the Italian population from 2 to 20 years of age, taken as a whole, or separately in two geographical areas (CentralNorth Italy and South Italy). See here for software and PDF.
Denmark
Updated in 2014, these include references for height/length, weight and head circumference from 0 to 20 years of age.
Sweden
Updated in 2002, these include references for height/length, weight and head circumference from 0 to 10 years of age.
Syndromespecific 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.