The line between typical and atypical development can be a hazy one. There are standards that pediatricians, physical therapists, and developmental experts use to monitor growth and deviations from the norm, which allow us to recommend interventions when appropriate. In 2006, the World Health Organization (WHO) released a whole new set of standards for evaluating and assessing the development of children from birth to 5 years.
What makes this new standard a great tool to monitor the change and growth of infants? This standard is based on data collected from healthy children, over multiple years, in six diverse geographic regions including Southeast and Southwest Asia, Europe, West Africa, North and South America. What is exciting about the new evaluation tool is that now, pediatric specialists have more than just reference curves for physical growth, but curves for motor development as well.
The six gross motor milestones WHO examined in babies were the following:
The “windows of milestone achievement” were organized into percentile rankings which pediatricians and physical therapists can use, much like a growth chart.
Without delving too deep into statistics and calculations, the typical age range (in months) for each milestone is listed below:
1. Sitting without support: 3.8 – 9.2 months
2. Standing with assistance: 4.8 – 11.4 months
3. Hands-and-knees crawling: 5.2 – 13.5 months
4. Walking with assistance: 5.9 – 13.7 months
5. Standing alone: 6.9 – 16.9 months
6. Walking alone: 8.2 – 17.6 months
The average (mean) age for healthy children achieving each milestones is as follows:
1. Sitting without support: 6 months (with 1.1 month standard deviation, SD)
2. Standing with assistance: 7.6 months (with 1.4 month SD)
3. Hands-and-knees crawling: 8.5 months (with 1.7 month SD)
4. Walking with assistance: 9.2 months (with 1.5 month SD)
5. Standing alone: 11 months (with 1.9 month SD)
6. Walking alone: 12.1 months (with 1.8 month standard deviation)
What is most interesting is that about 90% of the children studied met their milestones in a common sequence, and only 4% of the children skipped hands-and-knees crawling. (Read here about the importance of crawling.)
As you read over these standards and timelines, remember that every baby develops differently from another. If you see your baby fall behind on any of the 6 gross motor milestones above, mention it to his pediatrician, and she will most likely recommend a physical therapist to help him along.
WHO Multicentre Growth Reference Study Group. WHO Motor Development Study: Windows of achievement for six gross motor development milestones. Acta Paediatrica, 2006; Suppl 450: 86-95.