Is my baby behind? Milestone windows, not averages

31 August 2026 · 7 min read · by Aleksandr Zhadko, developer of Rostik and dad of two

Almost every milestone chart a parent meets shows one number. Sits at six months. Walks at twelve. First word at one year. Then your baby turns seven months, isn't sitting, and the number becomes a verdict.

The number is an average, and an average is the least useful thing you can know about a baby. What matters is the window — the span between the earliest healthy babies and the latest ones. Those windows are enormous, and almost nobody shows them to you.

How wide the windows really are

The World Health Organization followed around 800 healthy, well-nourished children across five countries and recorded when each of six motor milestones appeared. They published not averages but windows of achievement: the range covering the middle 99% of normal children.

Milestone	Window (WHO)	What that means
Sitting without support	3.8 – 9.2 months	A baby sitting at 8½ months is inside the normal range, not late
Standing with assistance	4.8 – 11.4 months	Nearly seven months of spread
Hands-and-knees crawling	5.2 – 13.5 months	And roughly 1 in 20 healthy babies never crawls at all — they shuffle, roll, or go straight to walking
Walking with assistance	5.9 – 13.7 months	Cruising the furniture counts
Standing alone	6.9 – 16.9 months	Ten months wide
Walking alone	8.2 – 17.6 months	The famous “walks at 12 months” is the middle of a range nine months across

Source: WHO Multicentre Growth Reference Study, motor development windows of achievement.

Read that walking row again. A baby walking at sixteen months and a baby walking at nine months are both ordinary. If you had only the average, one of those families would have spent seven months frightened for no reason.

Why the charts you see look so much narrower

In 2022 the US Centers for Disease Control changed how their milestone checklists work. They stopped listing what 50% of babies do at each age and started listing what 75% do. The stated reason was to stop parents being told to “wait and see” when a child genuinely needed help.

It's a sensible change for spotting problems early. But it has a side effect nobody warns you about: a checklist built on the 75% mark means that one in four perfectly healthy babies will “miss” any given item on the day you read it. Miss a few and the list starts to look like a diagnosis. It isn't one. It's a screening tool doing exactly what it was designed to do — catch a wide net early — being read as a report card.

Two things that genuinely shift the whole picture, and are easy to forget: prematurity — milestones are measured from the due date, not the birth date, usually until age two, so a baby born eight weeks early is “ten months old” at twelve months; and practice — babies who spend little time on their front reach rolling and crawling later on average, which is a positioning effect, not a developmental one.

The early ones parents worry about most

Two milestones cause more 2 a.m. searching than all the rest combined, because they arrive before any professional appointment and feel like a referendum on whether your baby likes you.

Milestone	Typical	Worth mentioning if…
Social smile	6 – 8 weeks	no smile in response to you by 10 weeks (corrected for prematurity)
Following your face with their eyes	6 – 8 weeks	not fixing on faces, or eyes not moving together, by 3 months
Babbling with consonants	6 – 9 months	no babbling at all by 12 months
Responding to their own name	7 – 12 months	no response by 12 months — and get hearing checked first
Pointing at things to show you	12 – 15 months	no pointing or showing by 18 months
First recognisable words	10 – 15 months	no words by 18 months, or fewer than 50 by 2 years

Note what the third column is doing. It isn't “your baby is late”. It's the point at which the wait-and-see answer stops being the right one and a conversation becomes worthwhile. Those two things are very different, and the gap between them is where most parental panic lives.

What actually warrants a call, at any age

Here is the part worth screenshotting, because it's much shorter than parents expect. Age-based checklists are noisy. These signs are not, and they matter more than any missed item on a list:

Losing a skill they already had — words, waving, eye contact, sitting. Regression at any age is the single clearest reason to be seen promptly.
Consistent one-sidedness — always reaching with the same hand before 18 months, or one arm or leg moving noticeably less than the other.
Floppiness or stiffness — a baby who feels limp when lifted, or whose legs cross and stiffen.
No reaction to loud sounds at any age, or no response to their name by 12 months.
Eyes that don't work together after 3 months, or an eye that turns in or out persistently.
Not walking at all by 18 months.
Your own gut. Parental concern is itself a recognised risk factor in developmental screening. If you're worried and can't say why, that counts — say so out loud at the appointment.

Signs drawn from NHS guidance on baby development and CDC developmental milestones. This is information for parents, not a diagnosis — assessment belongs to your health visitor, GP or paediatrician.

If you're in the UK: what the reviews actually are

Health visitors in England review children at roughly 9–12 months and again at 2–2½ years, often using a questionnaire you fill in beforehand — commonly an Ages & Stages Questionnaire (ASQ-3). Parents routinely arrive at these appointments believing the questionnaire is a test their child can fail.

It isn't scored like an exam. It's a screening instrument with deliberately generous thresholds, designed to over-refer rather than miss anyone. Scoring below the cut-off in one domain doesn't mean a diagnosis; it means a second look, and often the second look is the end of it. If your child has just been referred on the back of one, the most useful thing you can bring to the next appointment is not worry but specifics: what they do, when, and how it has changed over the last two months.

The comparison that actually helps

Not your baby against the average, and not your baby against your friend's baby. Your baby against your baby, eight weeks ago.

Development in the first two years is lumpy. Long plateaus, then a week where three things arrive at once. Judged day to day, a plateau feels like stalling; judged across two months, it's almost always visible progress. That comparison is the one clinicians actually make, and it's the one parents can't make from memory — because the previous version of your baby is genuinely hard to recall once the new one has arrived.

Rostik keeps the “eight weeks ago” version

Log a milestone in a tap when it happens, and Rostik holds the dates alongside sleep, feeds and growth on WHO percentile curves — so “has anything changed?” has an answer, and the health visitor gets specifics instead of a guess. It also carries the routine vaccination schedule for 12 countries, including the NHS one.

Try Rostik — free for 7 days