Executive summary In 1993 the World Health Organization (WHO)undertook a comprehensive review of the uses and interpretation of anthropometric references.The review concluded that the NCHS/WHO growth reference,which had been recommended for international use since the late 1970s,did not adequately represent early childhood growth and that new growth curves were necessary.The World Health Assembly endorsed this recommendation in 1994.In response WHO undertook the Multicentre Growth Reference Study (MGRS)between 1997 and 2003 to generate new curves for assessing the growth and development of children the world over. The MGRS combined a longitudinal follow-up from birth to 24 months and a cross-sectional survey of children aged 18 to 71 months.Primary growth data and related information were gathered from 8440 healthy breastfed infants and young children from widely diverse ethnic backgrounds and cultural settings (Brazil,Ghana,India,Norway,Oman and USA).The MGRS is unique in that it was purposely designed to produce a standard by selecting healthy children living under conditions likely to favour the achievement of their full genetic growth potential.Furthermore,the mothers of the children selected for the construction of the standards engaged in fundamental health-promoting practices,namely breastfeeding and not smoking. This report presents the first set of WHO Child Growth Standards(i.e.length/height-for-age,weight- for-age,weight-for-length,weight-for-height and body mass index(BMD)-for-age)and describes the methodical process followed in their development.The first step in this process was a consultative expert review of some 30 growth curve construction methods,including types of distributions and smoothing techniques to identify the best approach to constructing the standards.Next was the selection of a software package flexible enough to allow the comparative testing of the alternative methods used to generate the growth curves.Then the selected approach was applied systematically to search for the best models to fit the data for each indicator. The Box-Cox-power-exponential(BCPE)method,with curve smoothing by cubic splines was selected for constructing the WHO child growth curves.The BCPE accommodates various kinds of distributions,from normal to skewed or kurtotic.The age-based indicators originating at birth required a power-transformation to stretch the age scale (x-axis)as a preliminary step to fitting the curves.For each set of curves,the search for the best model specification began by examining various combinations of degrees of freedom to fit the median and variance estimator curves.When data had a non-normal distribution,degrees of freedom for parameters to model skewness and kurtosis were added to the initial model and adequacy of fit evaluated.Apart from length/height-for-age,which followed a normal distribution,the other standards required the modelling of skewness,but not kurtosis.The diagnostic tools used iteratively to detect possible model misfits and biases in the fitted curves included various tests of local and global goodness of fit,worm plots and residual plots. Patterns of differences between empirical and fitted percentiles were also examined,as were proportions of observed versus expected percentages of children with measurements below selected percentiles The methodology described above was followed to generate-for boys and girls aged 0 to 60 months -percentile and z-score curves for length/height-for-age,weight-for-age,weight-for-length,weight- for-height and BMI-for-age.The last standard is an addition to the set of indicators previously available as part of the NCHS/WHO reference.In-depth descriptions are presented of how each sex- specific standard was constructed.Also presented are comparisons of the new WHO standards with the NCHS/WHO growth reference and the CDC 2000 growth charts. To interpret differences between the WHO standards and the NCHS/WHO reference it is important to understand that they reflect differences not only in the populations used,but also in the methodologies applied to construct the two sets of growth curves.To address the significant skewness of the NCHS/WHO sample's weight-for-age and weight-for-height,separate standard deviations were xvii-- xvii - Executive summary In 1993 the World Health Organization (WHO) undertook a comprehensive review of the uses and interpretation of anthropometric references. The review concluded that the NCHS/WHO growth reference, which had been recommended for international use since the late 1970s, did not adequately represent early childhood growth and that new growth curves were necessary. The World Health Assembly endorsed this recommendation in 1994. In response WHO undertook the Multicentre Growth Reference Study (MGRS) between 1997 and 2003 to generate new curves for assessing the growth and development of children the world over. The MGRS combined a longitudinal follow-up from birth to 24 months and a cross-sectional survey of children aged 18 to 71 months. Primary growth data and related information were gathered from 8440 healthy breastfed infants and young children from widely diverse ethnic backgrounds and cultural settings (Brazil, Ghana, India, Norway, Oman and USA). The MGRS is unique in that it was purposely designed to produce a standard by selecting healthy children living under conditions likely to favour the achievement of their full genetic growth potential. Furthermore, the mothers of the children selected for the construction of the standards engaged in fundamental health-promoting practices, namely breastfeeding and not smoking. This report presents the first set of WHO Child Growth Standards (i.e. length/height-for-age, weight￾for-age, weight-for-length, weight-for-height and body mass index (BMI)-for-age) and describes the methodical process followed in their development. The first step in this process was a consultative expert review of some 30 growth curve construction methods, including types of distributions and smoothing techniques to identify the best approach to constructing the standards. Next was the selection of a software package flexible enough to allow the comparative testing of the alternative methods used to generate the growth curves. Then the selected approach was applied systematically to search for the best models to fit the data for each indicator. The Box-Cox-power-exponential (BCPE) method, with curve smoothing by cubic splines was selected for constructing the WHO child growth curves. The BCPE accommodates various kinds of distributions, from normal to skewed or kurtotic. The age-based indicators originating at birth required a power-transformation to stretch the age scale (x-axis) as a preliminary step to fitting the curves. For each set of curves, the search for the best model specification began by examining various combinations of degrees of freedom to fit the median and variance estimator curves. When data had a non-normal distribution, degrees of freedom for parameters to model skewness and kurtosis were added to the initial model and adequacy of fit evaluated. Apart from length/height-for-age, which followed a normal distribution, the other standards required the modelling of skewness, but not kurtosis. The diagnostic tools used iteratively to detect possible model misfits and biases in the fitted curves included various tests of local and global goodness of fit, worm plots and residual plots. Patterns of differences between empirical and fitted percentiles were also examined, as were proportions of observed versus expected percentages of children with measurements below selected percentiles. The methodology described above was followed to generate ─ for boys and girls aged 0 to 60 months ─ percentile and z-score curves for length/height-for-age, weight-for-age, weight-for-length, weight￾for-height and BMI-for-age. The last standard is an addition to the set of indicators previously available as part of the NCHS/WHO reference. In-depth descriptions are presented of how each sex￾specific standard was constructed. Also presented are comparisons of the new WHO standards with the NCHS/WHO growth reference and the CDC 2000 growth charts. To interpret differences between the WHO standards and the NCHS/WHO reference it is important to understand that they reflect differences not only in the populations used, but also in the methodologies applied to construct the two sets of growth curves. To address the significant skewness of the NCHS/WHO sample's weight-for-age and weight-for-height, separate standard deviations were