The ranges, from the study that measured them

Most milestone charts give a single age. The WHO Multicentre Growth Reference Study did something more useful: it followed babies across five countries and published, for six gross motor milestones, the window between the 1st and 99th percentile — that is, the span that covers almost every normally developing child.

Sitting without support: 3.8 to 9.2 months
Standing with assistance: 4.8 to 11.4 months
Hands-and-knees crawling: 5.2 to 13.5 months
Walking with assistance: 5.9 to 13.7 months
Standing alone: 6.9 to 16.9 months
Walking alone: 8.2 to 17.6 months

Read the last line again. A baby walking at eight months and a baby walking at seventeen months are both inside the normal range. The study's own note is that "the windows represent normal variation" and that they "vary in width" — the narrowest is sitting without support at 5.4 months, and the widest are standing alone at 10.0 months and walking alone at 9.4 months.

Nothing about a chart that gives "walking: 12 months" survives contact with that data.

Crawling is optional

Here is the finding that settles a very common worry. In the WHO study, "some 35 children (4.3%) were never observed to crawl on hands and knees". They were not delayed; they used something else. The paper notes that "other studies also report that this milestone is sometimes not performed and that instead some other type of locomotion is used, such as bottom shuffle or crawling on the stomach", both of which were seen in the study itself.

The sequence is also less fixed than people assume. "In about 90% of the cases, the pattern observed followed a fixed sequence for five of the milestones... with only hands-and-knees crawling shifting in between the earlier milestones." Crawling is the one that moves around, and sometimes it does not turn up at all.

What this looks like on the CDC checklists

The CDC's revised checklists list what most children (75% or more) do by a given age — a deliberately higher bar than the old 50th-percentile lists, so a missing item is worth mentioning.

By 6 months: rolls from tummy to back, pushes up with straight arms when on their tummy, leans on hands to support themselves when sitting.
By 9 months: gets to a sitting position by themselves, sits without support, moves things from one hand to the other, uses fingers to rake food towards themselves.
By 1 year: pulls up to stand, walks holding on to furniture, drinks from a cup without a lid as you hold it, picks things up between thumb and pointer finger.

Note what is absent. "Crawls" is not on the nine-month list, and "walks alone" is not on the twelve-month list. Both are consistent with the WHO windows above, and both are routinely misreported elsewhere as deadlines.

The CDC is also explicit about the limits of its own lists: they "are not developmental guidelines or standards for children's development" and "should not be used as screening or diagnostic tools to detect developmental delays".

Babies born prematurely

The NHS is unambiguous: "If your baby was born prematurely, their developmental age will be calculated from your original due date, not from the actual date they were born, until they are 2 years old."

A baby born at 30 weeks is ten weeks younger than the calendar suggests, and expecting them to pull to stand on a term timetable manufactures a problem that does not exist. Work out the number that matters with our adjusted age calculator, then read the ranges above against that age.

What is not just variation

Wide ranges do not mean nothing is ever worth raising. The signals that change the conversation:

A skill your baby had and has stopped doing. The CDC asks families directly whether their baby "has lost any skills he/she once had".
Persistent asymmetry — consistently using one hand, one arm or one leg far more than the other, or dragging one side.
A baby who feels very stiff or very floppy when handled.
Not sitting without support by around nine months, given the WHO window closes at 9.2 months.
Not walking independently well past eighteen months, given the walking-alone window closes at 17.6 months.
Your own persistent unease. The CDC's line is "you know your child best. Don't wait."

When to speak to a health visitor or doctor

The NHS builds in a checkpoint at exactly the right age: the nine-to-twelve-month review, preceded by an Ages and Stages Questionnaire (ASQ-3) you complete at home. Bring the red book and bring the specifics. Contact your health visiting team or GP between reviews for any of the signs above — the CDC's advice is to ask about developmental screening, and if concern persists, to ask for a referral to a specialist and to contact early intervention services.

What actually helps

Floor time, mostly. The CDC's nine-month tips are practical: "place toys on the ground or on a play mat a little out of reach and encourage your baby to crawl, scoot, or roll to get them", and "put your baby close to things that she can pull up on safely". Its four-month advice adds the other half: "let your baby have time to move and interact with people and objects throughout the day. Try not to keep your baby in swings, strollers, or bouncy seats for too long."

Then baby-proof, because mobility arrives before judgement. The CDC's list: move sharp or breakable things out of reach, lock away medicines, chemicals and cleaning products, and keep the poison helpline number in every phone. Bare feet or grippy socks on smooth floors help more than shoes, which are for outdoors and protection rather than for learning to walk.

Crawling, Standing and First Steps