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WHO has recently published the first set of results from its Multicentre Growth Reference Study (MGRS). The full version is available on the web at www.who.int/childgrowth/standards/technical_report/en/index.html and a series of related papers has been published in Acta Paediatrica 2006; Suppl 450:7-101 – details have already been mentioned on this website.
IFM presents three papers concerning the report, all of which have been prepared by members of IFM’s Advisory Committee on Child Health and Nutrition:
- Introduction and Summary
- Implications for the Developed World and Comparisons with Other Growth Charts
- Implications for Developing Countries
Part I: Introduction and Summary
The WHO report is necessarily a lengthy document (312 pages, 136 figures, 97 tables and almost 40 references). The aim of this commentary is to supply a brief introduction to and summary of the document, but clearly this can be no substitute for first-hand reading of the report. Direct quotations from the report are shown in italics and any other comments are in standard type face.
Structure of the report
The report consists of an executive summary (3 pages), Introduction (2 pages), Methods (11 pages, six of which are on statistical methods); throughout the report there is extensive presentation of statistical methods and models that only those with expert knowledge of statistics and auxology will be able to comment on and consider their validity. The bulk of the report then follows with four main sections:
- length/height for age,
- weight for age,
- weight for length/height,
- body mass index for age.
Each of these main sections is divided by sex, and for each sex there are the following subsections:
- sample,
- size,
- results,
- comparison of these results/standards with NCHS and CDC charts.
Finally, in each subsection there is a comparison of the results in boys with those in girls.(As yet there are no published data on head circumference from this study, which most paediatricians would require when making an assessment of growth in an individual child. However, the head circumference data will be published later.)
The report finishes with a Conclusion and a reference list.
Some extracts from the report
(Note: These are not necessarily in the order in which they appear in the report; they are presented to fit in with the conventional order of introduction, methods, results and discussion.)
Growth charts are an essential component of the paediatric tool kit. Their value resides in helping determine the degree to which physiological needs for growth and development are met in an individual during childhood. Beyond their usefulness in assessing children's nutritional status, many governmental and United Nations agencies rely on growth charts to measure the general well-being of populations, formulate health and related policies, and plan interventions and monitor the effectiveness.
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 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.
Methods
The MGRS combined a longitudinal follow-up from birth to 24 months and a cross-sectional survey of children aged 18 months to 6 years that included 8440 healthy, breastfed infants and young children from widely diverse ethnic backgrounds and cultural settings (Brazil, Ghana, India, Norway, Oman and USA).
The study populations lived in socioeconomic conditions favourable to growth and where mobility was low, greater than 20% of mothers followed WHO feeding recommendations and breastfeeding support was available. Individual inclusion criteria were:
- no known health or environmental constraints to growth, mothers willing to follow MGRS feeding recommendations (i.e. exclusive or predominant breastfeeding for at least 4 months). [Note: current WHO recommendations are exclusive breast feeding for 6 months.]
- introduction of complementary foods by the age of 6 months, (current WHO recommendations are no complementary foods before the age of 6 months) and continued partial breastfeeding for up to at least 12 months, no maternal smoking before and after delivery, single term birth, and absence of significant morbidity.
- term low-birth-weight (less than 2500 g) infants (2.3%) were not excluded. Since it is likely that in well-off populations, such infants represent small but normal children, their exclusion would have artificially distorted the standards' lower percentiles.
- eligibility criteria for the cross-sectional component were the same as those for the longitudinal component with the exception of infant feeding practices. A minimum of three months of any breastfeeding was required for participants in the study's cross-sectional component (among these children, exclusive breast feeding was not essential at any age, and some children were no longer breast fed after 3 months of age).
To avoid the influence of unhealthy weights for length/height, observations falling above +3 SD and below -3 SD of the sample median were excluded prior to constructing the standards. For the cross-sectional sample, the +2 SD cut-off (i.e. 97.7 percentile) was applied instead of +3 SD as the sample was exceedingly skewed to the right, indicating the need to identify and exclude high weights for height. This cut-off was considered to be conservative given that various definitions of overweight all apply lower cut-offs than the one used (a matter for expert statisticians and auxologists).
Results
In each section, after detailed statistical considerations, and for each sex, the results are presented as charts: smoothed centile curves, z score charts, comparison with NCHS and CDC values; and also as tables.
Discussion and Comments
Differences between the previously recommended NCHS/WHO international reference and the new WHO standards have been illustrated in this report.
As expected, there are notable differences that vary by age, sex, anthropometric measure and specific percentile or z-score curve. Differences are particularly important in infancy. Impact on population estimates of child malnutrition will depend on age, sex, anthropometric indicator considered, and population-specific anthropometric characteristics. A noteworthy effect is that estimates of stunting will be higher throughout childhood when assessed using the new WHO standards compared to the previous international reference. The growth pattern of breastfed infants compared to the NCHS/WHO reference will result in a substantial increase in underweight rates during the first half of infancy (i.e. 0-6 months) and a decrease thereafter. For wasting, the main difference between the new standards and the old reference is during infancy (i.e. up to about 70 cm length) when wasting rates will be substantially higher using the new WHO standards. With respect to overweight, use of the new WHO standards will result in a greater prevalence that will vary by age, sex and nutritional status of the index population.
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.
The MGRS is unique in that it was purposely designed to produce a standard rather than a reference. Although standards and reference both serve as a basis for comparison, each enables a different interpretation. Since a standard definition of how children should grow, deviations from the pattern it describes are evidence of abnormal growth. A reference, on the other hand, does not provide as sound a basis for such value judgments, although, in practice, references often are mistakenly used as standards (this suggests 'a standard of excellence' rather than a ‘standard for reference’ and may attract comment as these results are examined further by expert bodies).
The MGRS data provide a solid foundation for developing a standard because they are based on healthy children living under conditions likely to favour achievement of their full genetic grow potential. Furthermore, the mothers of the children selected for the construction of the standard engaged in fundamental health-promoting practices, namely breastfeeding and not smoking.
A second feature of the study that makes it attractive as a basis for an internationally applicable standard is that it included children from a diverse set of countries: Brazil, Ghana, India, Norway, Oman and the USA. By selecting privileged, healthy populations, the study reduced the impact of environmental variation. Assessment of differences in linear growth among the child populations in the MGRS shows a striking similarity among the six sites, with only about 3% of variability in length being due to differences among sites compared to 70% due to differences among individuals. Thus, excluding any site has little effect on the 3rd, 50th, and 97th percentile values, and pooling data from all sites is entirely justified. The remarkable similarity in growth during early childhood across human populations is consistent with genomic comparisons among diverse continental groups reporting a high degree of inter-population homogeneity. Nevertheless, the MGRS sample has considerable built-in ethnic or genetic variability in addition to cultural variation in how children are nurtured, which further strengthens the standards' universal applicability. (The lack of ethnic differences is important, and is an argument for not having race/ethnic specific growth charts, as exist in some countries.) A key characteristic of the new standards is that they explicitly identify breastfeeding as the biological norm and establish the breastfed child as the normative model for growth and development. (Note that some of the infants studied were never exclusively breast fed, but all of them were partially breast fed).
Another distinguishing feature of the new standards is that they include windows of achievement for six, gross, motor-developmental milestones that are presented elsewhere. Although WHO issued recommendations concerning attained physical growth in the past, it had not previously made any recommendations for assessing motor development. (The reference for this statement is given as Acta Paediatrica 2006,Suppl 450:39-47.)
The growth standards presented in this report provide a technically robust tool that represents the best description of physiological growth for children under five years of age. The standards depict normal early childhood growth under optimal environmental conditions and can be used to assess children everywhere, regardless of ethnicity, socioeconomic status and type of feeding.
This material has been prepared by B. Wharton on behalf of the IFM's Advisory Committee on Child Health and Nutrition, September 2006.
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