WHO Motor Development Study: Windows of achievement for six gross motor development milestones

To review the methods for generating windows of achievement for six gross motor developmental milestones and to compare the actual windows with commonly used motor development scales.

As part of the WHO Multicentre Growth Reference Study, longitudinal data were collected to describe the attainment of six gross motor milestones by children aged 4 to 24 mo in Ghana, India, Norway, Oman and the USA. Trained fieldworkers assessed 816 children at scheduled visits (monthly in year 1, bimonthly in year 2). Caretakers also recorded ages of achievement independently. Failure time models were used to construct windows of achievement for each milestone, bound by the 1st and 99th percentiles, without internal demarcations.