A 2006 WHO study set wide age windows for six motor milestones

The play

Sitting up, crawling, standing, walking: these are the milestones a baby carrier, a walker toy, or an activity gym is often marketed against by month. The most widely cited scientific source behind those month ranges is not a toy company at all but the World Health Organization's Multicentre Growth Reference Study, whose motor development component followed 816 children in Ghana, India, Norway, Oman, and the United States from age 4 to 24 months, with trained fieldworkers assessing each child at scheduled visits and caretakers independently recording when each skill appeared.

What the evidence says

The study, published in a 2006 Acta Pædiatrica supplement, reports 'windows of achievement' bound by the 1st and 99th percentiles for six gross motor milestones. The narrowest window, sitting without support, ran from about 3.8 to 9.2 months; the widest, walking alone, ran from about 8.2 to 17.6 months. About 90 percent of children followed a common sequence across the six milestones, and 4.3 percent skipped hands-and-knees crawling entirely. WHO's own indicator page, as retrieved 16 September 2026, republishes these same windows as its current reference tables for gross motor development.

Age fit and safety

The authors state their own intended use directly: the windows are 'recommended for descriptive comparisons among populations, to signal the need for appropriate screening when individual children appear to be late in achieving the milestones, and to raise awareness about the importance of overall development.' That is a screening trigger, not a diagnostic cutoff, and the study's sample, while spanning five countries, was drawn from families meeting specific health and feeding criteria rather than an unselected general population.

What to look for

This is an editorial application of the study's own caution. A baby outside a window on one milestone, especially near its edge, is inside normal variation as this data describes it; the study's authors built the ranges specifically to include that spread, not to exclude it. A toy or gym marketed to a narrow single month misrepresents how wide these WHO-documented windows actually are.

• Is a product's stated age range narrower than the WHO window for that same skill?
• Has a child been well outside a window's late edge, which the study frames as a screening trigger?
• Is 'on track' being judged against one milestone or the overall six-milestone sequence?

The WHO windows are among the most carefully documented age ranges in child development, and their own text argues against precision they were never built to offer: normal motor development in this data is a wide band, not a single month.