WHO Growth Charts, Part 2

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:

1. Introduction and Summary
2. Implications for the Developed World and Comparisons with Other Growth Charts
3. Implications for Developing Countries
   

Part II: Implications for the developed world and comparisons with other growth charts

The WHO Multicenter Growth Reference Study (MGRS) that resulted in the recently released WHO Child Growth Standards reflects an impressive effort by all concerned with the study. The growth standards are based on data from close to 8500 young children, most of whom were breast-fed exclusively or predominantly for at least 4 months. They were from diverse ethnic backgrounds and cultural settings and lived under conditions favoring achievement of their full genetic growth potential. In addition to being breast-fed exclusively or predominantly for at least the first 4 months of life or longer, they received complementary feedings by 6 months of age. Moreover, their mothers did not smoke or engage in other unhealthy activities.

The standards are based on longitudinal data from about 1750 children from birth to 24 months of age plus cross-sectional data from another approximately 6700 children from 18 to 71 months of age. Exclusion of 85% of the 13741 mother/infant pairs screened for the longitudinal component and 69% of the 21,510 mother/infant pairs screened for the cross-sectional component illustrates the rigidity of the selection process.

The WHO growth standards, indeed, are standards rather than simply references. They describe how children raised as described grow rather than how children living in a specific place at a specific time without dietary specifications grew. Thus, if growth of the WHO population is accepted as “optimal”, deviations from the pattern of growth documented by the WHO standards are evidence of abnormal growth not simply growth that differs from that of the population standard. References are often used interchangeably with standards but they do not provide as sound a basis for value judgments as do standards. A standard is based on a prescriptive rather than simply a descriptive approach. The new WHO growth standards describe the growth of healthy children living under prescribed conditions deemed to support achievement of their full genetic growth potential. In contrast, the NCHS/WHO growth reference was based on the growth of predominantly formula-fed infants living in the mid-western United States in the 1970s without specific dietary guidance and without regard for living circumstances.

Differences between the new WHO growth standards and the NCHS/WHO growth reference reflect differences in the populations as well as differences in the methods used to generate the growth curves. The more recent CDC growth charts, which are widely used in the United States and elsewhere, were generated using the same methods used to generate the new WHO growth standard; thus, differences between the new standards and the CDC references reflect primarily differences in the populations on which the respective standard or reference is based.

The statistical methods used in creating the new WHO growth standards are quite sophisticated. Many, in fact, are beyond my level of comprehension and this is likely to be so for many others as well. Nonetheless, the statisticians involved in this exercise are recognized internationally for their expertise in analyzing growth data and generating growth curves. This, of course, doesn’t guarantee the validity of the methods but it helps to reassure those of us with less statistical expertise. The roughly 330-page report describing the MGRS concentrates on the statistical methods. While this is necessary for such a scientific undertaking, the details are likely to be “over-the-head” of most users of the new standards.

A key characteristic of the new growth standards is that they explicitly identify breastfeeding as the biological norm and establish the breast-fed child as the normative model for growth and development. Certainly, it is difficult to refute the assumption that breastfeeding is the biological norm. If so, it follows that the breast-fed infant is the normative model for growth and development. Further, most recent dietary recommendations for infants are based on the content of various nutrients in breast milk and the mean volume of breast milk ingested by infants of different ages.

The extent to which the new WHO growth standards will be utilized by pediatricians, nutritionists and public health officials is not clear. On the one hand, the release of the new WHO growth standards has received considerable attention and has generated considerable curiosity about the new standards. On the other hand, although not particularly evident in 2000 when the CDC references were introduced to replace the NCHS/WHO growth references in the United States, there is likely to be some resistance to abandoning a familiar reference.

Another potential barrier to adoption of the new growth standards is the fact that it is difficult from the published data and curves to identify the differences between these standards and currently used references. I have attempted to illustrate differences by calculating Z-scores for weight-for-age and length (height)-for age of children at the median weight and length (height) of the WHO population based on the CDC means and standard deviations as well as the Z-scores for weight-for-age and length (height)-for-age of children at the median weight and length (height) of the CDC population based on the new WHO mean and standard deviation values (see Tables 1 and 2). I also have plotted the median length (height), weight, and BMI of boys from the CDC population on the new WHO growth curves (Figures 1-3). These exercises demonstrate that the differences between the two vary with age as well as other variables. In general, however, the length of boys comprising the WHO population is about the same or a little greater than that of the CDC population depending on age and other factors. The median weight of the CDC population, on the other hand, is greater than that of the WHO standard from about 9 to 36 months of age. The median BMI of boys comprising the CDC population also is higher than that of boys comprising the WHO population.

Since values less than the 5th percentile and greater than the 95th percentile are considered abnormal, comparison of the 5th and 95th percentile values of the various anthropometric indices as well as the median values of WHO and CDC populations at various ages also is helpful. These comparisons for both boys and girls are shown in Tables 3-6. As the report states, use of the new WHO standards vs. the NCHS/WHO reference will increase estimates of stunting throughout childhood, will increase estimates of under-weight during the first half of infancy (to about 6 months of age) and decrease such estimates thereafter, will increase estimates of wasting during infancy and also will increase estimates of the prevalence of obesity (high BMI).

These differences in estimates of stunting, under-weight, wasting and obesity based simply on use of a different standard or reference may create problems for monitoring improvements in or worsening of nutritional status of children in developing and developed countries. The obvious question is which estimate represents the true estimate of malnutrition? Unfortunately, since the differences between the new standards and the CDC references vary with age, gender, anthropometric measure and other factors, it is not possible to provide an algorhythm that converts prevalence values based on previous references to those based on the new standards or vice versa.

Despite the scientific excellence of the MGRS and the production of growth standards based on growth of primarily breast-fed infants living in environments conducive to achieving full genetic growth potential, the few differences between the new standards and currently used references are not easy to grasp (at least for me). These differences are complex and differ by age, gender, anthropometric index and other factors. More effort on interpreting the new standards for potential users is necessary before potential users completely embrace the new standards.

Table 1

Z-Scores (SDs) of Weight/Age and Length (Height)/Age
of WHO Polulation Based on CDC Means and SDs

	Weight/Age			Length (Height)/Age
Age (m)	Boys	Girls		Boys	Girls
0	-0.42	-0.41		-1.10	-1.12
1	-0.57	-0.57		-0.79	-0.67
2	-0.10	-0.20		-0.50	-0.41
3	0.01	-0.08		-0.27	-0.27
4	-0.05	-0.05		-0.13	-0.17
5	-0.15	-0.08		-0.09	-0.16
6	-0.28	-0.18		-0.10	-0.16
9	-0.56	-0.53		-0.12	-0.18
12	-0.78	-0.78		-0.14	-0.13
18	-0.74	-0.81		-0.03	-0.03
24	-0.40	-0.51		0.07	0.09
30	-0.18	-0.24		0.15	0.10
36	-0.06	-0.02		0.21	0.22
42	-0.01	0.06		0.22	0.32
48	-0.01	0.10		0.19	0.38
54	-0.04	0.12		0.18	0.38
60	-0.08	0.07		0.18	0.30

Table 2

Z-Scores (SDs) of Length (Height)/Age, Weight/Age, and
BMI/Age of CDC Population Based on WHO Medians and SDs

	Length (Height)/Age			Weight/Age			BMI/Age*
Age (m)	Boys	Girls		Boys	Girls		Boys	Girls
0	0.06	0.07		0.37	0.36
1	0.98	0.82		0.66	0.6
2	0.59	0.50		0.15	0.15
3	0.32	0.31		0.02	0.02
4	0.16	0.21		0.05	0.02
5	0.11	0.17		0.14	0.08
6	0.11	0.17		0.26	0.18
9	0.17	0.19		0.58