Hania is five and a half months old. She is lying on a mat on the living-room floor, propped up on straight arms, watching the cat stroll past just out of her reach. Her grandmother, visiting for the afternoon, fetches two pillows from the bedroom. “She needs sitting up — her cousin Ola was sitting at this age,” she says, and arranges the pillows in a horseshoe. Hania, placed in the middle, looks surprised for a moment, then slowly tips to one side and stays there, cheek on the pillow. Her parents exchange a look: is it time? Should they be practising? Or is something wrong?
The answer to “when do babies sit up” is calmer than comparisons with cousins suggest. This article walks through the whole of motor development in the first eighteen months — from head control, through rolling, commando crawling, sitting and crawling, to first steps — based on a World Health Organization (WHO) study, the milestone checklists of the US Centers for Disease Control and Prevention (CDC) and parent guidance from the American Academy of Pediatrics (AAP).
When do babies sit up?
Most babies start sitting without support between six and nine months. In the WHO study, half of the children sat on their own at around 6 months, and almost all healthy children reached this skill between 3.8 and 9.2 months. The CDC lists sitting without support and getting into a sitting position unaided among the skills most children have by 9 months. The AAP writes that at 6–8 months a baby placed upright stays sitting without leaning forward on their arms.
This answer hides two different skills that are easy to confuse. The first is staying in a sitting position when someone puts the baby there. The second is getting into sitting on their own, from lying down or from all fours. The second usually comes later, and it is the one that really shows a baby “can sit”. We will come back to this difference, because it answers the question Hania’s parents were asking themselves: should you sit a baby up before they get there alone?
Two maps of development: WHO windows and CDC checklists
When you look up the age at which a baby “should” be able to do something, you find numbers that seem to contradict each other: one source says 6 months, another says 9. Both are true, because they describe different things.
WHO windows of achievement. As part of its Multicentre Growth Reference Study, the World Health Organization followed 816 healthy children in Ghana, India, Norway, Oman and the United States from 4 to 24 months of age. Trained fieldworkers visited the children monthly in the first year and every two months in the second. The result is a set of so-called windows of achievement for six motor skills: the range from the 1st to the 99th percentile, that is, the age at which the first one per cent of children reach a skill and the age by which 99 per cent have it. The WHO stresses that the whole window represents normal variation among healthy children and deliberately does not divide it into “early” and “late”.
CDC checklists. In 2022 the CDC, together with the AAP, revised its milestone checklists. Each skill was placed at the age by which at least 75% of children have it (Zubler et al., Pediatrics 2022). The authors explain why they did not use the median: a list based on the age at which half of children have a skill encourages a “wait and see” approach, since by definition half of children do not have it yet. Parents of children with disabilities told CDC focus groups that diagnosis was delayed because they had been told that “every child develops differently”.
The difference matters in practice. The middle of a WHO window tells you when a skill appears in a typical child. The position on a CDC checklist tells you when its absence is a good reason to talk to a doctor. If your baby is not sitting yet at six months, they are in the middle of the WHO window and before the CDC threshold — there is no reason to worry. If they are not sitting at ten months, they are past the CDC threshold and outside the WHO window — a good moment to talk to the paediatrician.
| Skill | Typical age (WHO median) | WHO window (1st–99th percentile) | CDC checklist (≥75% of children) |
|---|---|---|---|
| Holds head up when on tummy | — | — | by 2 months |
| Holds head steady when held | — | — | by 4 months |
| Rolls from tummy to back | — | — | by 6 months |
| Sits without support | 5.9 mo | 3.8–9.2 mo | by 9 months |
| Stands with assistance (holding furniture) | 7.4 mo | 4.8–11.4 mo | pulls up to stand: by 12 months |
| Crawls on hands and knees | 8.3 mo | 5.2–13.5 mo | not on the checklist |
| Walks holding on to furniture | 9.0 mo | 5.9–13.7 mo | by 12 months |
| Stands alone | 10.8 mo | 6.9–16.9 mo | — |
| Walks alone | 12.0 mo | 8.2–17.6 mo | a few steps: by 15 months; walks without holding on: by 18 months |
The WHO studied only six skills, from sitting upwards, so the first rows of the table rely on the CDC alone. Crawling, in turn, is not on the CDC checklists — and the WHO study itself explains why.
When do babies hold their head up?
Head control develops over the first four months, in two stages. First, a baby lifts their head while lying on their tummy: the CDC lists this among the skills most babies have by 2 months, and the AAP writes that most two-month-olds can lift their head when lying on their tummy. By 4 months, according to the CDC, a baby holds their head steady without support when you are holding them.
The AAP reminds parents that a newborn can turn their head to the side but has little neck control in the first few weeks and needs your hand under their head. Neck and trunk muscles get stronger every day, and the best training is time spent on the tummy.
Tummy time from the first days. The AAP recommends placing your baby on their tummy while they are awake and you are with them, from the day you bring them home from hospital: two to three times a day for 3–5 minutes to begin with, gradually longer, working up to 15–30 minutes a day by 7 weeks. For sleep, a baby always goes on their back — “back to sleep, tummy to play”, as the title of the AAP guide puts it. According to the AAP, tummy time builds the neck and trunk strength needed for the next motor milestones.
Many babies protest at first when they are on their tummy. The AAP suggests lying down facing them or placing a toy within their reach. By 4 months, according to the CDC, a baby on their tummy pushes up onto elbows and forearms, and by 6 months onto straight arms — just like Hania in the opening scene.
When do babies roll over from back to tummy and tummy to back?
Most babies start rolling over at around five months. The AAP says the skill usually appears at that age, that most babies roll from tummy to back before the other way round — and that the reverse order is normal too. By the end of the 4–7 month period, a baby usually rolls both ways. The CDC places rolling from tummy to back on its 6-month checklist.
The first roll from tummy to back often happens by accident: the baby pushes hard on one arm and “tips over” onto their back, clearly surprised. Rolling the other way takes a twist of the trunk and a swing of the leg, so it is already a deliberate movement.
From the first roll, a baby can reach the edge of a bed or changing table within seconds. The floor is the safest place for them to play.
How can you support rolling? For a six-month-old, the CDC suggests a simple game: put your baby on their tummy or back with a toy just out of reach, and encourage them to roll over to get it. There is no need to roll the baby yourself. All they need is a reason to move and a place where they can do it.
When do babies start commando crawling?
Commando crawling — moving forward on the tummy using arms and legs, which Polish parents call “pełzanie” — is often described as a separate stage before crawling on all fours. In the sources this article relies on, however, it has no date or age window of its own. Neither the WHO nor the CDC includes it among the skills they measure. It appears in them as one of several ways babies get around before crawling, or instead of it.
The AAP writes that a few children never crawl, and instead scoot on their bottoms or slither on their stomachs. The WHO study recorded the same: some children, instead of crawling, bottom-shuffled or crawled on their stomachs. Among activities for a nine-month-old, the CDC suggests placing toys on the floor or a play mat a little out of reach and encouraging the baby to crawl, scoot or roll to get them — listing these ways side by side, with no hierarchy.
In practice, some babies start commando crawling soon after they learn to roll and do it for weeks before lifting their tummy off the floor. Others do it briefly or not at all and go straight to all fours. Still others pivot in circles or roll across the whole room. None of these variants is a cause for concern in itself. The AAP points to what matters: whether the baby coordinates both sides of the body and uses each arm and leg equally.
What helps commando crawling most is the floor: firm, clean and safe, without a deep, soft rug that hands sink into. And time — long stretches in which the baby is on their tummy and has a reason to move.
How babies learn to sit — sitting step by step
Sitting is a big change for a baby: for the first time both hands are free and they see the world from above. No wonder parents want to hurry this moment along. It is worth knowing, though, how sitting really develops.
The stages of sitting
The AAP describes sitting as a process. At first a baby, once placed in sitting, leans forward and props themselves on straight arms — the so-called “tripod” sit, in which the arms act as two extra legs. The CDC lists leaning on hands to support themselves when sitting among the skills most babies have by 6 months. Over time the hands are no longer needed: the AAP says that at 6–8 months a baby placed upright stays sitting without leaning on their arms.
In the WHO study, “sitting without support” meant something specific: the child sits up straight with the head erect and does not use arms or hands to balance or to support the position. Half of the children reached this at around 6 months, and the window ran from 3.8 to 9.2 months. It is the narrowest of the six windows in the whole study — sitting is a skill that appears at a relatively predictable time in healthy children.
The last stage is getting into sitting unaided. The CDC lists it separately: by 9 months most babies “get to a sitting position by themselves”. A baby might get there, for example, from lying on their side, by pushing up on a forearm and then a hand, or from all fours — shifting the hips to one side and landing in sitting. At that point sitting is truly theirs for the first time: they can get into it when they want and out of it when they are tired.
Sitting when placed versus sitting up alone
This difference is the heart of the whole topic. A baby sat up by an adult, even if they can stay there, cannot yet get out of the position. When they get tired, they topple — sideways or backwards — and wait for someone to pick them up. A baby who gets to sitting on their own has gone through rolling, pushing up on their arms, pivoting and often commando crawling or all fours along the way. Each of these positions works the muscles of the trunk, shoulders and hips, and teaches the baby how to catch themselves when they lose balance.
In other words, sitting up alone is not a single skill but the culmination of months of work on the floor. Sitting a baby up early does not shorten that road. It gives them the view from above, but not the tools to get there and safely back down.
Should you sit a baby up before they can do it alone?
It is worth separating honestly what the medical sources say from what the free-movement approach proposes.
The medical sources do not forbid sitting a baby up. In its guide to movement at 4–7 months, the AAP writes that while a baby is learning to balance in sitting, you can hold them up or support their back with pillows. Among activities for a six-month-old, the CDC suggests holding your baby up while they sit, so they can look around and look at toys while they learn to balance. According to these sources, then, short moments of supported sitting with you beside the baby are nothing to worry about.
The same sources agree on something else: a baby needs plenty of time for free movement and little time held still in equipment. The AAP advises limiting time in freestanding swings, bouncy chairs and car seats. In its tips for a four-month-old, the CDC says to give your baby time to move and interact with people and objects throughout the day, and not to keep them in swings, strollers or bouncy seats for too long.
The free-movement approach, close to Montessori education, goes a step further and suggests not sitting a baby up at all until they sit up alone — instead giving them the floor and time until they get there themselves. This is a parenting choice, not a medical recommendation, and you will not find it in AAP or CDC guidance. Its logic, though, is simple and follows from the difference between assisted and independent sitting: every hour spent in a sitting position the baby cannot get into alone is an hour not spent practising on the floor the things that lead to sitting.
A sensible compromise for most families looks like this: the floor as the default play space, short moments of supported sitting on your lap or at the table if the baby is interested, and no “wedging” with pillows in which a baby spends long minutes unable to change position. Hania’s grandmother meant well. Hania, tipping onto the pillow, simply showed that she was not ready yet.
How to support sitting at home
- Plenty of time on the tummy and on the side. These are the positions from which a baby later pushes up into sitting.
- Toys a little out of reach. Reaching, pivoting and shuffling build trunk strength better than sitting in one place.
- A firm floor rather than a soft sofa. On a stable surface a baby can push off and prop themselves up.
- Comfortable clothes. Bodysuits and trousers that do not restrict hips and knees.
- Patience with a “wobbly” sit. First sits are unsteady, with hands on the floor — that is how it should be.
When do babies crawl, and does every baby need to crawl?
Most babies crawl on hands and knees in the second half of their first year, but not all of them. In the WHO study, half of the children crawled at 8.3 months, and the window ran from 5.2 to 13.5 months. The AAP says crawling is usually mastered between 7 and 10 months. At the same time, 35 of the 816 children in the WHO study, or 4.3%, never crawled on hands and knees — and these were healthy children who went on to stand and walk like the others.
In WHO terms, crawling means alternately moving forward or backward on hands and knees, with the tummy off the floor, in at least three consecutive movements. The crawling window is wider than the sitting window, and the order varies: the WHO study found that about 90% of children reached five of the milestones in a common sequence, with crawling “moving around” between them. The largest group (42%) crawled before standing with support, but more than a third (36%) stood holding on first and only then crawled. One child in twelve (8.5%) started crawling only after already walking along furniture.
That explains why the CDC, in its 2022 revision, did not put crawling on any milestone checklist. A skill that some healthy children skip is not suitable as a warning sign — provided the baby gets around the floor some other way and uses both sides of the body. The AAP says it plainly: as long as your baby is coordinating each side of their body and using each arm and leg equally, there is no cause for concern.
A crawling baby needs space and safety. The CDC advises giving your baby safe places to explore: moving sharp or breakable things out of reach and locking away medicines and cleaning products. The AAP recommends gates at the top and bottom of the stairs. It is also worth lying down on the floor and looking at your home from a baby’s height.
Pulling to stand and cruising along furniture
Around the time of crawling, a baby begins to pull up to stand. According to the WHO, half of the children stood with assistance — holding on to furniture with both hands, with their weight on their legs — at 7.4 months, within a window of 4.8 to 11.4 months. The next step is cruising, walking sideways along the sofa or table: the WHO median is 9.0 months, the window 5.9 to 13.7. The CDC lists pulling up to stand and walking while holding on to furniture among the skills most children have by their first birthday.
The AAP describes this period briefly: the baby pulls up to stand at every opportunity. At first, many babies do not know how to sit back down and call for help, standing at the cot rail. Instead of sitting them down yourself, you can show them the movement — gently bend their knees so they slide down onto their bottom. After a few tries they will do it alone.
Cruising needs furniture that can take it. The AAP points out that whatever the baby holds on to should have no sharp edges and be properly weighted or attached to the floor. A bookcase that could be pulled over should be fixed to the wall before the baby starts pulling up.
Standing alone comes later. In the WHO study, half of the children stood without holding on to anything at 10.8 months, and the window was very wide: from 6.9 to 16.9 months. A wide window simply means that healthy children differ a great deal here.
When do babies start walking?
Most babies take their first independent steps around their first birthday, but the normal range is wide. In the WHO study, half of the children walked alone at 12 months, and almost all healthy children started walking between 8.2 and 17.6 months. The CDC says that by 15 months most children take a few steps on their own, and by 18 months walk without holding on to anyone or anything. The AAP reminds parents that starting earlier or later is normal.
The WHO counted a child as walking when they took at least five steps independently, upright, with no contact with a person or object. First steps therefore often come earlier than what the researchers counted as “walking”, and — as the AAP puts it — they are shaky. A baby walks with legs wide apart and arms up, sits down with a bump and gets up again. That is the normal picture for the first weeks.
The walking window is one of the two widest in the WHO study — more than nine months across. A one-year-old who is already running and an eighteen-month-old who is just letting go of the furniture may both be developing equally well.
How can you support first steps? Above all with space: a clear floor, stable furniture to cruise along, and a push toy or a chair the child pushes ahead of themselves — which is not the same as a baby walker, as explained below. When your child starts walking outdoors, the AAP recommends shoes that are closed-toe, comfortable and flexible, with non-slip soles. According to the AAP, a child does not need inserts, high backs, reinforced heels or special arches. Check the size monthly, because feet grow quickly at this stage.
Less equipment, more floor
A baby’s room is easy to fill with equipment that promises development: bouncers, swings, seats that hold a baby sitting, jumpers, walkers. Some of them can be useful for a few minutes while you cook dinner. The problem starts when minutes turn into hours.
The logic is the same as with sitting. Any equipment that holds a baby in one position takes away time on the floor, where they practise rolling, commando crawling, pushing up and sitting. That is why the AAP and the CDC both advise limiting time in swings, bouncy seats, strollers and car seats.
Baby walkers: why the AAP says no
Baby walkers with wheels are a separate matter. The American Academy of Pediatrics has called for a ban on their manufacture and sale. In its guidance for parents, the AAP writes that walkers send thousands of children to hospital every year, that most injuries happen when children roll down stairs, and that a child in a walker can move more than 3 feet (about 90 cm) in one second. Most injuries happen while an adult is watching — they simply cannot react in time.
Despite the name, walkers do not teach children to walk. The AAP writes that they can actually delay when a child starts walking alone. As safer alternatives, the AAP lists stationary activity centres without wheels, play yards and high chairs. In the context of this article, though, the best alternative remains the floor and a stable piece of furniture the baby pulls up on by themselves.
Free movement in the Montessori approach
This section is an extra: everything that matters most — tummy time, the floor, little equipment — already follows from AAP and CDC guidance. Montessori education, however, offers a coherent explanation of why to do it this way.
The Association Montessori Internationale describes the development of movement in the first years as an internal drive a child is born with. From birth, a child is driven to develop voluntary movement and — as the nervous system matures and the child interacts with the environment — lifts their head, rolls over, crawls and finally stands. In this view, walking is the most obvious sign of independence: the child chooses where to go and how to get there. From birth, a child needs order, which provides reference points, and freedom of movement to explore.
In this approach, the adult’s role is to prepare the environment and remove obstacles, not to push development along. At home this usually means a few simple things: a mat or blanket on the floor as a permanent play space, a few toys within sight and a little out of reach, low, stable furniture to pull up on, and as little equipment as possible that holds the baby in one position. For more on how to set up such a room without buying out the shop, see the article on the Montessori child’s room.
The floor bed often comes up in this context, but it is a separate topic. For a baby, the rule the AAP opens its tummy time guide with comes first: always on the back for sleep. Free movement is about awake time, not sleep.
When should you see a doctor or physiotherapist?
The CDC advises not to wait and to talk to your child’s doctor if your child is not meeting one or more milestones for their age, has lost skills they once had, or you have other concerns. In that conversation, it is worth asking about developmental screening. The CDC reminds parents that they know their child best, and the AAP adds that parents’ observations are an important part of a paediatrician’s evaluation.
The signs worth discussing are listed directly by the AAP and the CDC:
- your baby has difficulty holding their head and neck steady (AAP),
- is not sitting by themselves at nine months (AAP; the CDC places sitting without support on its 9-month checklist),
- struggles to roll over, sit or walk (AAP),
- is not using a part or one side of the body (AAP),
- seems too floppy, or their muscles seem stiff (AAP),
- jitters or shakes too much (AAP),
- stops doing something they used to do — at any age (AAP, CDC),
- has a hard time staying balanced or walks in an unusual way (AAP),
- is not walking alone at 18 months (CDC; in the WHO study, 99% of children walked alone before 17.6 months).
The WHO windows serve the same purpose. The study’s authors recommend them as a signal that a child who appears late with a given skill should be appropriately screened — not as a verdict, but as a reason to check.
Babies born early. The AAP notes that children born before 37 weeks of pregnancy may develop later than their peers, and advises using adjusted age for them: subtract the number of weeks your baby was born early from their actual age in weeks. An eight-week-old born three weeks early has an adjusted age of five weeks. The AAP recommends using adjusted age until the second birthday; by then most children have caught up to the typical range.
Paediatric physiotherapist. The AAP writes that a child with delayed gross motor skills, such as rolling over, sitting or walking, may be referred to a physical therapist. In Poland, the usual first step is the paediatrician, who will assess development and decide on further assessment. If you are wondering which signs in slightly older children — asymmetry, frequent tripping, W-sitting — are worth checking and what such a visit looks like, the Polish article on when to see a paediatric physiotherapist covers it. Paediatric physiotherapy is also described on the physiotherapy page (in Polish).
FAQ — parents’ questions about baby motor development
When do babies sit up? Most babies sit without support between 6 and 9 months. In the WHO study, half of the children sat on their own at around 6 months, and the window was 3.8–9.2 months. The CDC lists sitting without support and getting into a sitting position unaided among the skills most children have by 9 months.
When do babies hold their head up? By 2 months most babies lift their head when lying on their tummy, and by 4 months they hold it steady without support when held (CDC). The best training is awake tummy time, from the first days after coming home from hospital (AAP).
When do babies roll from back to tummy? The AAP says rolling usually appears at around 5 months, most often from tummy to back first and then from back to tummy, although the reverse order is normal too. By the end of the 4–7 month period most babies roll both ways.
When do babies roll from tummy to back? Usually as the first direction, at around 5 months (AAP). The CDC places rolling from tummy to back on the list of skills most babies have by 6 months.
When do babies start commando crawling? Commando crawling on the tummy has no separate date in WHO, CDC or AAP sources — it appears in them as one of the ways babies get around before crawling or instead of it. Some babies commando crawl for weeks, others briefly or not at all. What matters is that the baby uses both sides of the body equally (AAP).
When do babies crawl? In the WHO study, half of the children crawled at 8.3 months, within a window of 5.2–13.5 months, and the AAP gives a range of 7–10 months. About 4% of healthy children in the WHO study never crawled. Crawling is not on the CDC checklists either.
When do babies start walking? Half of the children in the WHO study walked alone at 12 months, and the window was 8.2–17.6 months. According to the CDC, by 15 months most children take a few steps on their own, and by 18 months walk without holding on to anything.
Does a baby walker help a child learn to walk? No. The AAP writes that walkers do not teach children to walk and can delay independent walking, and that every year they injure thousands of children, most often through falls down stairs. The AAP has called for a ban on the manufacture and sale of baby walkers with wheels.
Final thoughts
Motor development in the first eighteen months is a long journey across the floor: from lifting the head on the tummy, through rolling, commando crawling and sitting, to crawling, pulling up on furniture and first steps. The WHO windows show how wide the normal range is, and the CDC checklists show when a missing skill is a good reason to talk to a doctor. Between the two there is room for every child’s own pace.
Hania sat up on her own six weeks after her grandmother’s visit, from all fours, shifting her hips to one side — and immediately reached for the toy lying next to her. She did not need pillows, just the floor and time. Walking usually comes around the first birthday, roughly when many families start thinking about nursery; we cover when to enrol in nursery in Poland and from what age a child can attend separately. MOC Montessori runs a nursery for children from one to two and a half years and a preschool with one mixed-age group, linked by the Montessori method and educational continuity under one roof. Specialists from a partner counselling centre come to MOC Montessori, so paediatric physiotherapy takes place on site. For more about everyday life in the youngest group, see the Montessori nursery page.