Aim: To assess differences in length/height among populations in the WHO Multicentre Growth Reference Study (MGRS) and to evaluate the appropriateness of pooling data for the purpose of constructing a single international growth standard.
Methods: The MGRS collected growth data and related information from 8440 affluent children from widely differing ethnic backgrounds and cultural settings (Brazil, Ghana, India, Norway, Oman and the USA). Eligibility criteria included breastfeeding, no maternal smoking and environments supportive of unconstrained growth. The study combined longitudinal (birth to 24 mo) and cross-sectional (18-71 mo) components. For the longitudinal component, mother-infant pairs were enrolled at delivery and visited 21 times over the next 2 y. Rigorous methods of data collection and standardized procedures were applied across study sites. We evaluate the total variability of length attributable to sites and individuals, differences in length/height among sites, and the impact of excluding single sites on the percentiles of the remaining pooled sample.
Results: Proportions of total variability attributable to sites and individuals within sites were 3% and 70%, respectively. Differences in length and height ranged from -0.33 to +0.49 and -0.41 to +0.46 standard deviation units (SDs), respectively, most values being below 0.2 SDs. Differences in length on exclusion of single sites ranged from -0.10 to +0.07, -0.07 to +0.13, and -0.25 to +0.09 SDs, for the 50th, 3rd and 97th percentiles, respectively. Corresponding values for height ranged from -0.09 to +0.08, -0.12 to +0.13, and -0.15 to +0.07 SDs.
Conclusion: The striking similarity in linear growth among children in the six sites justifies pooling the data and constructing a single international standard from birth to 5 y of age.