How milestones are actually defined
In 2022, the CDC and the American Academy of Pediatrics rebuilt the milestone checklists from the evidence up (Zubler et al., 2022). The key change: each milestone is now placed at the age when 75% or more of children can do it — not the average age. That's deliberate. An average is the age half of children haven't reached yet, which made the old lists great at causing worry and bad at guiding action. The new framing means:
Two honest consequences. If your child does things later than a friend's baby, that alone means nothing — most of the normal range sits after the average. But if your child hasn't reached a milestone by the checklist age, that's not a verdict either — it's simply the evidence-based moment to mention it to your pediatrician rather than wait.
Sitting, crawling, walking: the WHO windows
The best data on gross motor milestones comes from the WHO's Multicentre Growth Reference Study, which followed healthy children on five continents and published windows of achievement — the full age range in which healthy, typically developing children reached each skill (WHO, 2006). The windows are strikingly wide:
So one healthy baby walks at nine months and another at seventeen, and both are inside the published windows. The same study found the order varies too — and that about 4% of perfectly healthy children skip hands-and-knees crawling altogether (WHO, 2006). Crawling style is a personality fact, not a report card.
Milestone highlights by age
Selected highlights from the CDC's 2022 evidence-informed checklists — each item is something most children (75% or more) can do by that age. These are excerpts; every age links to the full CDC checklist, which is the version to actually use.
By 2 months 0–2 mo
- Smiles when you talk to or smile at them
- Calms down when spoken to or picked up
- Watches you as you move
- Holds head up when on tummy
- Makes sounds other than crying
By 6 months half a year
- Takes turns making sounds with you
- Laughs; knows familiar people
- Reaches to grab a toy they want
- Rolls from tummy to back
- Pushes up with straight arms on tummy
By 9 months screening visit
- Sits without support
- Looks when you call their name
- Smiles or laughs at peek-a-boo
- Babbles strings like "mamamama"
- Looks for objects dropped out of sight
By 1 year 12 mo
- Pulls up to stand; walks holding furniture
- Waves "bye-bye"
- Calls a parent "mama"/"dada" or a special name
- Looks for things they see you hide
- Picks things up between thumb and pointer finger
By 18 months screening visit
- Walks without holding on
- Tries to say three or more words besides "mama"/"dada"
- Points to show you something interesting
- Follows one-step directions without gestures
- Scribbles; copies you doing chores
By 2 years 24 mo
- Says at least two words together, like "more milk"
- Points to things in a book when you ask
- Uses gestures beyond waving, like blowing a kiss
- Kicks a ball; runs
- Eats with a spoon
By 3 years preschool age
- Talks with you with at least two back-and-forth exchanges
- Talks well enough for others to understand, most of the time
- Says first name when asked
- Draws a circle when you show them how
- Notices other children and joins them to play
By 4 years pre-K
- Says sentences with four or more words
- Names a few colors of items
- Pretends to be something else during play
- Draws a person with three or more body parts
- Catches a large ball most of the time
By 5 years off to school
- Counts to 10
- Tells a story they heard or made up, with two or more events
- Keeps a conversation going, three or more exchanges
- Hops on one foot; writes some letters of their name
- Follows rules or takes turns in games
Language: the widest "normal" of all
Early vocabulary has the biggest healthy spread of any skill. When researchers measured hundreds of children with standardized vocabulary inventories, the normal range at any given age was enormous — some typical 18-month-olds say a handful of words while others say hundreds (Fenson et al., 1994). That's why the checklists track communication, not word counts: pointing to show you things, following directions, taking turns with sounds, and combining words ("more milk") by around age two matter more than the size of the word list.
The strongest lever is free: back-and-forth conversation — the serve-and-return loops described on our early development page. Narrate, pause, and let your child answer in babble; every exchange counts as a turn.
"Act early" beats "wait and see"
The entire point of the 2022 revision is that a missed milestone should trigger a conversation, not a panic and not a shrug (Zubler et al., 2022). The evidence-based moves:
- Ask for the standardized screening at the 9, 18, and 30 month well-child visits — it's an AAP recommendation, not an extra (Lipkin & Macias, 2020).
- If a checklist milestone hasn't appeared by its age, mention it at the next visit — early support works best early.
- If your child loses skills they used to have — words, gestures, social interest — talk to your pediatrician promptly, at any age.
- Track along at home with the CDC's free checklists and Milestone Tracker app.
- You know your child best. "I'm probably overthinking this" is still worth saying out loud to a professional.
And the flip side, just as evidence-based: a child on the late edge of a WHO window, or who skipped crawling, or whose cousin talked earlier, is not behind. The ranges are the science.
How TwinkleHush lines up with the milestones
None of our content accelerates milestones, and we'll never claim it does. What we make is matched to the windows above: high-contrast printable cards for the months when vision is developing fastest, audio-first sleep long-plays for the nights in between, and slow, one-idea learning videos only from age two — the age the screen research actually supports.
The same science, as Shorts
Parent-facing, a minute long, study cited on screen. More arrive as we make them.
Common questions, answered from the research above
When do babies start walking?
Healthy babies in the WHO's Multicentre Growth Reference Study first walked alone anywhere from 8.2 to 17.6 months (WHO, 2006). Nine months and seventeen months are both inside the published window. The CDC checklist places "walks without holding on" at 18 months — the age by which 75% or more of children do it.
Is it bad if my baby skips crawling?
No. About 4% of perfectly healthy children in the WHO study never did hands-and-knees crawling, and the order of motor milestones varied too — some stand before they crawl, others scoot or bottom-shuffle (WHO, 2006). Crawling style is a personality fact, not a report card.
What does it mean if my baby hasn't reached a milestone?
Since 2022 each CDC/AAP milestone sits at the age when 75% or more of children can do it (Zubler et al., 2022). Doing something later than a friend's baby means nothing on its own — most of the normal range is after the average. Not having a listed skill by its checklist age isn't a verdict either; it is the evidence-based moment to mention it at your next pediatric visit rather than wait.
When do babies start talking?
Language has the widest healthy spread of any skill: some typical 18-month-olds say a handful of words while others say hundreds (Fenson et al., 1994). The checklists therefore track communication rather than word counts — trying three or more words by 18 months, putting two words together ("more milk") by around 2 years, and pointing, following directions, and taking turns with sounds along the way.
When should I worry about my child's development?
Ask for the standardized screening at the 9, 18 and 30 month visits (Lipkin & Macias, 2020). Mention any checklist milestone that hasn't appeared by its age at the next visit. And if your child loses skills they used to have — words, gestures, social interest — talk to your pediatrician promptly, at any age. "I'm probably overthinking this" is still worth saying out loud to a professional.
Selected references
- Zubler JM, Wiggins LD, Macias MM, et al. (2022). Evidence-informed milestones for developmental surveillance tools. Pediatrics, 149(3), e2021052138.
- WHO Multicentre Growth Reference Study Group (2006). WHO Motor Development Study: windows of achievement for six gross motor development milestones. Acta Paediatrica, Suppl 450, 86–95.
- Lipkin PH, Macias MM; AAP Council on Children with Disabilities (2020). Promoting optimal development: identifying infants and young children with developmental disorders through developmental surveillance and screening. Pediatrics, 145(1), e20193449.
- Fenson L, Dale PS, Reznick JS, et al. (1994). Variability in early communicative development. Monographs of the Society for Research in Child Development, 59(5), 1–173.
- CDC, "Learn the Signs. Act Early." milestone checklists (cdc.gov/act-early) — 2022 revision, developed with the AAP.