You saw them from the side as they ran toward the swing: belly out, bottom pushed back, back arched like a little S drawn too hard. Your mother-in-law's voice came back to you saying "stand up straight". Tonight you typed four words into the search bar and ended up here.
Here is the answer up front: between ages 2 and 5 that look is the rule, not the exception, and it almost never concerns the back. What you see is mostly a question of proportions: a trunk that is still short, a full abdomen, an abdominal wall that does not yet act as a corset. It is not an out-of-range curve. The useful check is not the shape at rest, it is how they move. And there are signs that must not be let pass, which I will spell out plainly further on.
The curve you see is not bigger. It is the finding that surprises everyone
It seems logical that a young child should have a more pronounced lordosis than a teenager, and that they then "straighten out" as they grow. X-rays say the opposite.
A study of 151 children aged 3 to 15 with no musculoskeletal problems, divided into four age bands (3-6, 7-9, 10-12, 13-15), measured the alignment in the sagittal plane band by band. The global lordosis angle differed significantly between the groups, and the direction was this: with growth, total thoracic kyphosis and lumbar lordosis increased, with the increase in lordosis due mainly to the two lowest segments, those above the sacrum.
The confirmation comes from much larger numbers. A French national cohort of 1,059 healthy children compared sagittal parameters across five levels of skeletal maturity: the pelvic incidence went from 40° to 46°, the pelvic tilt from 4° to 9°, and L1-S1 lordosis from 51° to 56° (p < 0.001), with the peak of change in the second part of the pubertal growth spurt. The authors describe a cascade: first the shape of the pelvis changes, then the lordosis adapts, and last the thoracic kyphosis.
Put simply for you: your child's back is not in a phase of being "too curved" that has to flatten out. If anything it is the opposite. A protruding belly at three and a half is not the photograph of a faulty spine.
How wide "normal" is (much wider than you think)
A German study of 54 children aged 3 to 10 measured reference lumbopelvic values on standing X-rays. The mean lordosis was 47.7°, with a standard deviation of 11.2°. Eleven degrees of standard deviation means that two perfectly normal children can differ by twenty degrees. There were no differences between boys and girls.
Two limits must be stated, because they change how much weight the figure carries: these are 54 children from a single German paediatric orthopaedic hospital, and most of them (44 out of 54) were between 7 and 10 years old. For children aged 2-5, who are the ones we are talking about, the radiographic data are really scarce.
And the root of the problem is that this synthesis does not yet exist. In August 2025 the protocol of a systematic review was published, dedicated precisely to the normal values of kyphosis and lordosis from birth to adulthood: the authors state in black and white that there is variability in the normative values reported in the paediatric population, and that this variability may reflect the breadth of normality or depend on the different measurement and imaging techniques. A protocol is the declaration that the work is about to begin, not that it is finished.
So: anyone who tells you how many degrees a three-year-old "should" have is going well beyond what is known.
The problem with the label "hyperlordosis"
When the curve is not measured with an X-ray but looked at, in a photograph or by eye, the label becomes elastic.
A study of 467 children and teenagers aged 10 to 16 at a private school in Porto Alegre, Brazil, assessed lumbar hyperlordosis photographically. The prevalence came out at 78%.
Pause on that number. If a criterion classifies nearly eight out of ten teenagers as abnormal in a normal school population, the problem is not the teenagers: it is the criterion. And the same study is useful for a second reason. It found statistically significant associations with female sex (prevalence ratio 1.08; 95% CI: 1.03-1.13), with abdominal strength below the 20th percentile (1.10; 95% CI: 1.05-1.15) and with flexibility below the 20th percentile (1.07; 95% CI: 1.01-1.12). These are increases of the order of 7-10%: real in statistical terms, negligible in practical terms. The story of "weak abdominals cause the protruding belly" holds up much less well than it is told.
There is also an Australian prospective cohort that gives the definitive measure of how much of a minority the "right posture" is: among 1,373 adolescents followed from birth, at age 14 the classification of standing sagittal posture gave 29% neutral, 22% flat, 27% sway and 22% hyperlordotic. The "neutral" profile is the largest category, but it still covers fewer than one teenager in three. Variability is normality.
If the wider topic of posture interests you (backpack, desk, hours sitting), I have covered it with the numbers in School posture and backpacks: what really matters. And if the doubt is the opposite, the pelvis tilted backwards and a flat back, the picture is in Pelvic retroversion: when it is a problem.
So why does it look like this? Proportions
If the curve is not bigger, where does the look come from?
From three things that have nothing pathological about them. The first is that a young child's trunk is short compared with the rest, and the abdominal organs fit inside it with proportions different from yours: the same volume, in a lower and narrower space, sticks out. The second is that the abdominal wall does not yet work the way it will: it is not weakness to be corrected, it is a containment function that matures with the years and with movement. The third is that standing is still a skill under construction: a young child stands on a wide base, with the pelvis tilted forward, because that is more stable, and the protruding belly is the geometric consequence of that pelvis, not the cause.
Add to that the fact that at this age you see them from the side much more often, at the beach, in the pool, and you understand why this question arrives all at once for everyone.
It is the same reasoning as knees that knock or legs that look "bowed": variations that are watched over time, not corrected in a hurry, as I explain in Bow legs and knocking knees in children.
When weight is a factor (and at what age we know)
This part has to be stated precisely, because it is easy to apply it to the wrong child.
In the Australian cohort mentioned above, teenagers whose body mass index trajectories were high or rising from childhood showed at age 14 significantly higher proportions of hyperlordotic or sway posture compared with neutral posture. And a Polish study of 910 children aged 10-12 found that body mass index and fat mass correlated significantly with the shape of the lumbar lordosis and with the tilt of the body's axis, while thoracic kyphosis was similar between those of normal weight and those with excess weight.
Note the age: 10-14 years. These data say something about a pre-teen who is overweight; they say nothing about the belly of a three-year-old, which is not abdominal fat but body proportion. Applying those results to a kindergarten child is exactly the mistake that makes half of what is read online useless.
The red flags: when a protruding belly is not posture
Here everything changes, and it must be read even if the rest has reassured you. An arched back can be the compensation for muscle weakness, and in that case the signal is not the shape: it is the function.
Gowers' sign, the one that cannot be missed
Ask your child to sit on the floor and then to stand up. Watch how they do it.
A child with weakness of the muscles around the pelvis cannot get up in one movement: they roll onto their front, get onto all fours with their legs wide apart and climb back up by pressing their hands on their own thighs, pulling themselves up on their own body. This is Gowers' sign, and it must be named because it is the sign that must not be missed.
There is, however, something almost nobody says, and it comes from a video analysis of 33 patients with Gowers' sign, plus three healthy children (a small, descriptive series, to be taken for what it is): the classic, striking form is the late one. The earlier manifestations are much more discreet: simply a slower rise, one hand placed on the thigh, the four-point position before getting up, a wide base, a tendency to stand on the toes, the trunk bent forward in an exaggerated way. And in moderate weakness hip abduction, shifts in the tilt of the pelvis and lordosis appear.
Reread the last line: in weakness, lordosis is a compensation. That is why the protruding belly deserves thirty seconds of observation even when it is almost always nothing.
The practical threshold, without beating about the bush: if to get up off the floor they have to press their hands on their legs consistently (not once because they are tired, but every time), and if they struggle to climb stairs or to run like their peers, this is a matter for the paediatrician and not the physiotherapist, and it should not be waited out.
The first test is a blood test for CK (creatine kinase). A review dedicated to the early signs of Duchenne muscular dystrophy is clear: the diagnosis is often not made before age four, with a delay of up to two years from the onset of the first symptoms, and the best strategy is precisely to do that test early, since it is inexpensive and sensitive, in all boys who show signs of developmental delay and abnormal motor tests at routine paediatric visits. In a US series of 179 boys with Duchenne muscular dystrophy followed at a single centre, the mean age at diagnosis was 5.1 years (± 2), and the delay in starting to walk (at 16 months or later) was strongly associated with cognitive delay: when there is a global developmental delay, muscle weakness is often overlooked as the cause.
It is a rare condition and concerns almost only boys. But the cost of checking is a blood sample; the cost of not checking is years.
The other red flags
Ask for a medical assessment, without waiting, if alongside the belly there is:
- a waddling gait, that sways from side to side at every step, like a duck;
- real stiffness of the back or hip: they cannot bend forward, or one leg moves clearly less than the other;
- pain, especially if it occurs at night, if it is always in the same spot, or if it wakes them;
- limping, even slight, even only when tired. In paediatric literature limping is a diagnostic puzzle: the most common causes are trauma and infections, but inflammatory conditions and, though rare, cancers must also be considered;
- fever with refusal to bear weight on the leg: this is the combination to be checked the same day, not tomorrow. The paediatric review we cite uses the Kocher criteria: temperature above 38.5 °C, inability to bear weight, white blood cells above 12,000/mL and ESR above 40 mm/h. The more criteria are present, the higher the suspicion of septic arthritis;
- loss of a skill they had before: they used to walk and now walk worse, they used to climb stairs and no longer do.
And a rule that the same review repeats and that applies to you too: when something does not add up, look at the joint above and the one below the point they complain about. The authors recommend it thinking in particular of two hip conditions: slipped capital femoral epiphysis and Legg-Calvé-Perthes disease.
What is NOT needed at age three
Corrective postural exercises are not needed. There is no evidence that a corrective gymnastics programme changes the lumbar curve of a healthy young child, and it makes sense, since that curve is already changing on its own with growth, in a direction the exercises do not control.
There is no need to tell them "stand up straight". It gets two seconds of forced posture and a lasting message: there is something wrong with your body. It is a real cost against a benefit that has never been shown.
X-rays are not needed for a child who moves well, has no pain and has none of the red flags above.
Insoles, cushions, corsets or "postural insoles" bought to correct the profile of the belly are not needed.
There is no need to strengthen the abdominal muscles of a three-year-old to straighten their back: where it was measured, the association was too small to justify a programme.
What does make sense
Little, and that is not a consolation prize: it is what holds up under the evidence available.
What is needed is quantity and variety of movement, which is what truly builds trunk control: time on the floor, climbing, going up and down stairs alternating feet, uneven grass, sand, running and changing direction, fewer hours still in the pushchair or on a chair. What is needed is watching the trajectory, not the photograph: if six months later they do more things and do them better, they are within normal range whatever the side profile. And it helps, now and then, to have them do that getting-up-from-the-floor movement and see how they do it.
I should also state the limit, because that is fair: that this movement changes the degrees of lordosis is not proven. It is reasonable, it carries zero risk and it is good for a thousand other reasons. It is not a cure for something that is almost never a disease.
What we can do, and what we cannot
A child like this needs to be watched while moving, not standing still against a wall. This is one of the situations where a video call works well, because what needs to be seen are things you already have at home: how they walk down the hallway, how they run, how they climb stairs, how they get up from the rug.
With Ri-Hub Junior you start with a free 20-minute first consultation with a physiotherapist on the Italian professional register who works with children and teenagers: in most cases the result is that we tell you everything is fine and why, in others that it is better to see the paediatrician first. Up to age 10 sessions last 50 minutes and cost 59,90 €; from age 11 to 18 the standard session of 29,90 € applies.
And if behind it there is a complex neurological or neuromuscular condition, the right place is not us: those pathways require specialised centres and in-person assessments, and we prefer to tell you that straight away.
In the meantime, tonight, do just one thing: ask them to sit on the floor and then get up again. If they get up the way children usually do, you can stop looking at the protruding belly.
Sources
- The evolution of sagittal segmental alignment of the spine during childhood: Spine, 2005
- Spinal Sagittal Alignment Changes During Childhood: Results of a National Cohort Analysis of 1,059 Healthy Children: The Journal of Bone and Joint Surgery, 2023
- Normative values of the sagittal lumbopelvic alignment in children during the first decade of life: European Spine Journal, 2025
- Development of thoracic spine kyphosis and lumbar spine lordosis in the growing child from birth to adulthood: protocol for a systematic review: BMJ Open, 2025
- Lumbar hyperlordosis in children and adolescents at a private school in southern Brazil: occurrence and associated factors: Cadernos de Saúde Pública, 2012
- Trajectories of childhood body mass index are associated with adolescent sagittal standing posture: International Journal of Pediatric Obesity, 2011
- Does Excessive Body Weight Change the Shape of the Spine in Children?: Childhood Obesity, 2019
- Pathomechanics of Gowers' sign: a video analysis of a spectrum of Gowers' maneuvers: Clinical Orthopaedics and Related Research, 2012
- Detecting early signs in Duchenne muscular dystrophy: comprehensive review and diagnostic implications: Frontiers in Pediatrics, 2023
- Motor and cognitive delay in Duchenne muscular dystrophy: implication for early diagnosis: The Journal of Pediatrics, 2014
- The limping child: Pediatrics in Review, 2015
Last source check: September 2026.
