Assessment of sex differences and heterogeneity in motor milestone attainment among populations in the WHO Multicentre Growth Reference Study

Aim: To assess the heterogeneity of gross motor milestone achievement ages between the sexes and among study sites participating in the WHO Multicentre Growth Reference Study (MGRS). Methods: Six gross motor milestones (sitting without support, hands-and-knees crawling, standing with assistance, walking with assistance, standing alone, and walking alone) were assessed longitudinally in five of the six MGRS sites, namely Ghana, India, Norway, Oman and the USA. Testing was started at 4 mo of age and performed monthly until 12 mo, and bimonthly thereafter until all milestones were achieved or the child reached 24 mo of age. Four approaches were used to assess heterogeneity of the ages of milestone achievement on the basis of sex or study site. Results: No significant, consistent differences in milestone achievement ages were detected between boys and girls, nor were any site-sex interactions noted. However, some differences among sites were observed. The contribution of inter-site heterogeneity to the total variance was < 5% for those milestones with the least heterogeneous ages of achievement (hands-and-knees crawling, standing alone, and walking alone) and nearly 15% for those with the most heterogeneous ages of achievement (sitting without support, standing with assistance, and walking with assistance). Conclusion: Inter-site differences, most likely due to culture-specific care behaviours, reflect normal development among healthy populations across the wide range of cultures and environments included in the MGRS. These analyses support the appropriateness of pooling data from all sites and for both sexes for the purpose of developing an international standard for gross motor development.