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Gross Motor Milestones Explained

In brief

  • The source is a 2006 World Health Organization study: researchers followed 816 healthy children at five sites, in Ghana, India, Norway, Oman, and the United States, and recorded when each child first sat, stood, crawled, and walked.
  • What it found: a "window of achievement" for each of six gross motor milestones, from the 1st to the 99th percentile. For example, the study estimated that 98% of healthy children walk alone sometime between 8.2 and 17.6 months, and half are walking by 12.0 months.
  • What it does not establish: the study describes typical development in healthy, well-nourished children. It is not a diagnostic test, it does not define who needs therapy, and it says nothing about any therapy, DMI included. It was published in 2006, years before DMI existed.
  • What it means for a parent: a plain-language, evidence-based answer to "is that normal?" The normal range for each milestone is wide, and a child outside the window is a reason to ask for an assessment, not a verdict.

This explainer covers where the "normal" ages for gross motor milestones come from: the 2006 WHO Motor Development Study, a longitudinal study of 816 healthy children at five sites that produced the 1st to 99th percentile windows of achievement for sitting, standing, crawling, and walking. It is one piece of the evidence behind the phrase "a wide normal range" and a concrete way to understand typical development when a professional uses the term. It is not a diagnostic test, and it says nothing about any therapy.

What this study is

The paper is "WHO Motor Development Study: Windows of achievement for six gross motor development milestones," published in 2006 in Acta Paediatrica by the WHO Multicentre Growth Reference Study Group. It was part of the Multicentre Growth Reference Study, the same project that produced the WHO Child Growth Standards for length, height, and weight from birth to age 5. The motor development part set out to answer a simple question: at what ages do healthy children first sit, stand, crawl, and walk?

This matters because the windows give a published, international picture of the ages healthy children reach these milestones. They are one yardstick among several. The authors compared them with other commonly used scales, including the Denver II and the Bayley scales, and found the average ages mostly similar.

What the study did

Trained fieldworkers followed 816 children from about 4 to 24 months of age at five study sites, in Ghana, India, Norway, Oman, and the United States. Children were examined at scheduled visits, monthly in the first year and every two months in the second, and caregivers independently recorded when a new skill appeared between visits.

The researchers used failure-time, or survival, analysis to compute a "window of achievement" for each of six gross motor milestones. Each window runs from the 1st to the 99th percentile, estimated with a statistical model: the age range in which the model expects 98% of healthy children to reach that milestone. The group was healthy and well nourished by design; this is a description of typical development in favorable conditions, not a clinic sample and not children with diagnoses.

What it found

The milestones, each with the middle of the range (the 50th percentile) and the full 1st to 99th percentile window:

  • Sitting without support: middle of the range 5.9 months; window 3.8 to 9.2 months
  • Standing with assistance: 7.4 months; 4.8 to 11.4 months
  • Hands-and-knees crawling: 8.3 months; 5.2 to 13.5 months
  • Walking with assistance: 9.0 months; 5.9 to 13.7 months
  • Standing alone: 10.8 months; 6.9 to 16.9 months
  • Walking alone: 12.0 months; 8.2 to 17.6 months

About 90% of the children reached five of the milestones in the same order: sitting without support, standing with assistance, walking with assistance, standing alone, and walking alone. Hands-and-knees crawling was the one that moved around, showing up at different points among the earlier milestones. Another 77 children, 9.4% of the group, followed other orders, and 35 children, 4.3%, were never seen crawling on hands and knees; some got around in other ways, such as bottom shuffling or crawling on the stomach. Those 35 were left out of the crawling analysis. The authors conclude that the windows show the normal spread of ages at which healthy children reach these milestones, and they recommend using them to flag when a child who seems late should be screened.

What it does not tell you

  • It is not a diagnostic test. The windows describe what healthy, well-nourished children in favorable conditions did. A child outside a window is a reason to seek assessment, not a verdict on that child.
  • The range is deliberately wide. The 1st to 99th percentile window spans months, by construction. Even so, many children land close to the middle: half of the children in the study walked alone between 11.0 and 13.1 months.
  • It says nothing about children with motor delays, genetic conditions, or cerebral palsy, and nothing about therapy. Published in 2006, before DMI was introduced in February 2021, it cannot weigh in on whether any intervention changes the timing of milestones.
  • It does not define "motor delay" for eligibility purposes. Programs such as the Idaho Infant Toddler Program use their own criteria for who qualifies for early intervention services.

What it means for your child

Use the table as orientation, not as a stopwatch. If your child is still inside a window and has not reached that milestone yet, that is part of the normal variation the study found in healthy children. If your child is past the end of a window without the milestone, the authors treat that as a signal for screening. The next step is a conversation with your pediatrician and an assessment, ideally repeated over time. A child with a real delay deserves a careful evaluation, not a label.

The table also gives you a concrete way to talk with a therapist about motor delay, which is what DMI is used to treat. With the window in hand, you can ask which milestone, which window, and where your child falls in it.

What to ask

  • Ask which milestone range your pediatrician or therapist is using, and where your child currently falls in it. If the answer is not tied to a published window, ask what it is based on.
  • Ask whether your child has been assessed with a standardized measure, such as the GMFM-88, and for the youngest children the General Movements Assessment or the HINE, and when progress will be rechecked.
  • Ask what your state or insurance counts as "delay" for eligibility. In Idaho, the Infant Toddler Program serves children from birth to age 3 at no cost to families, and families can apply on their own or through their doctor.
  • If DMI is being discussed, ask how the therapist turns milestone work into measurable goals and how progress will be tracked, which the session guide covers in detail.

Where it appears on this site

Milestones run through most of this site. The What is DMI? guide describes DMI as working toward milestones like rolling, sitting, crawling, standing, and walking, and the DMI therapy guide repeats that framing. The Down syndrome guide discusses milestone timelines, the prematurity guide explains corrected age, and the session guide explains how progress is tracked. On the measurement side, the explainers on automatic postural responses, GMFCS levels, and the GMFM-88 show how motor function is actually scored.

Sources

Nothing here is medical advice, and milestone charts do not diagnose anything on their own. Discuss your child's development with your pediatrician or therapist.

About this site

Idaho DMI is an independent, non-commercial resource. It is not run by a clinic, it has no financial relationship with any provider listed here, and it accepts no advertising or referral fees. There is nothing to buy and no one selling you anything. The only form on this site asks families and therapists to tell us about DMI-certified providers we haven't found yet. It exists because Dynamic Movement Intervention is not well known in Idaho, and families whose children haven't progressed with conventional therapy deserve to know it's an option worth asking about.

Information here is drawn from DMI's official training organization and published clinical literature, and is linked to its sources so you can check it yourself. Nothing on this site is medical advice. Always talk with your child's pediatrician or therapist before starting a new therapy.

Last updated: September 2026