Original data study
How wide are typical baby development windows, really?
How this was built
The table below reproduces the World Health Organization's Multicentre Growth Reference Study (MGRS) Motor Development Study windows, the age range in which most typically developing children reach six major gross-motor milestones, alongside the study's reported median age. We did not run this study; we cite it and compute the width of each window (high minus low) here, since "how wide is normal" is the more useful and less cited number for a worried parent than the median alone.
Verified against the published WHO figures.
What the data shows
Milestones compared
6
Widest normal window
10 mo
Standing alone
Narrowest normal window
5.4 mo
Sitting without support
Average window width
7.9 mo
Every one of these six milestones has a normal window of at least 5.4 months, and the widest, standing alone, spans 10 months, nearly a year and a half of typical variation. That is the whole case for celebration over comparison: the "normal" age for any one of these is really a wide range, not a date.
Full data table
| Milestone | Typical window | Window width | Reported median | Source |
|---|---|---|---|---|
| Sitting without support | 3.8 to 9.2 months | 5.4 months | 5.9 months | WHO Motor Development Study (MGRS) |
| Standing with assistance | 4.8 to 11.4 months | 6.6 months | 7.6 months | WHO Motor Development Study (MGRS) |
| Hands-and-knees crawling | 5.2 to 13.5 months | 8.3 months | 8.5 months | WHO Motor Development Study (MGRS) |
| Walking with assistance | 5.9 to 13.7 months | 7.8 months | 9.2 months | WHO Motor Development Study (MGRS) |
| Standing alone | 6.9 to 16.9 months | 10 months | 11 months | WHO Motor Development Study (MGRS) |
| Walking alone | 8.2 to 17.6 months | 9.4 months | 12.1 months | WHO Motor Development Study (MGRS) |
Our own library's coverage
This site's week-by-week library currently covers 38 original age entries: 19 weekly entries from birth through week 24, and 19 monthly entries from 6 through 24 months. Every entry cites its sources and carries a reviewed date; see the full library.
Limitations
- The WHO MGRS study reflects a specific multi-country sample studied under specific conditions; it is the most widely cited motor-milestone dataset, not a universal guarantee for every child or context.
- Gross-motor windows are only part of development; language, social, and cognitive milestones each have their own typical ranges not captured in this specific table.
- This table describes typical ranges. It is not a screening tool. See the CDC note below for that.
Read the methodology behind our editorial process, or start with the weekly timeline tool.