Almost every family who comes to see me arrives holding a number. He’s on the third centile. She’s only 112 centimetres. The chart at school said he was below average. The number is where the worry starts, and it is almost never where the answer is.
Here is how I read a growth chart, in the order I read it.
First: what a centile is
A centile is a position in a queue.
If your child is on the 25th centile for height, it means that if you lined up 100 children of the same age and sex from shortest to tallest, your child would be standing at about position 25. Roughly a quarter are shorter; roughly three-quarters are taller.
That is the whole meaning. It is not a mark out of a hundred. There is no pass line, and there is certainly no prize for standing at the front of the queue. By definition, three children in every hundred are below the 3rd centile, and most of them are entirely healthy children who come from families of shorter people.
Second: the shape of the line
This is the part that matters, and it is the part a single measurement can never give you.
From roughly the age of two until puberty, healthy growth is remarkably loyal. A child settles onto a centile and travels along it year after year, like a train that has found its track. A child on the 10th centile at three who is still on the 10th centile at nine is, in growth terms, doing beautifully.
What draws attention is crossing. A line that drifts steadily downwards through the centile bands over two or three years is worth investigating even while the child is still comfortably “within the normal range”. Equally, a line that shoots upwards across bands outside of infancy or puberty deserves a look.
The direction is the signal. The position is context.
Third: does the height fit the family?
Short parents have short children. This is not a disappointing answer; it is a real, calculable one.
There is a rough arithmetic, the mid-parental height, that estimates the height range a child is genetically aimed at:
- For a boy: (mother’s height + father’s height + 13 cm) ÷ 2
- For a girl: (mother’s height + father’s height − 13 cm) ÷ 2
Give that figure roughly ±8–9 cm of leeway and you have the target range. A child sitting comfortably inside their family’s range, tracking a steady centile, is usually a child who needs reassurance rather than investigation.
A child growing well below their family’s range is a different conversation.
Fourth: does the height fit the weight?
The relationship between the two lines is one of the most informative things on the whole chart, and it is often the fastest route to a diagnosis.
| Pattern | What it often suggests |
|---|---|
| Weight falling first, height following later | Nutritional or gastrointestinal cause: coeliac disease, chronic illness, inadequate intake |
| Height falling while weight holds or rises | Endocrine cause: hypothyroidism, growth hormone deficiency, excess steroid |
| Both falling together, steadily | Chronic systemic illness; sometimes psychosocial |
| Both low but tracking steadily since infancy | Often familial or constitutional, and frequently normal |
This is exactly why I ask for the weight record too, and why I plot both before saying anything at all.
The four patterns that make me pay attention
- Crossing downwards through two centile bands after the age of two, over more than a year.
- A height well below the mid-parental range, particularly when the child’s growth rate is also slow.
- A growth rate under about 4–5 cm a year in a school-aged child between the ages of four and puberty.
- Height and puberty out of step. A very short child entering puberty early, or a child growing slowly with no pubertal signs well past the usual window.
None of these is a diagnosis. Each is a reason for a proper history, an examination and, usually, a small number of well-chosen tests.
What to bring me
The single most valuable thing you can bring to a first appointment is old measurements. Not a report. Measurements.
- The immunisation card, which usually has infant heights and weights
- School health records
- Any paediatrician’s file, however thin
- Pencil marks on a doorframe with dates written beside them
I am serious about the doorframe. Two dated marks eighteen months apart tell me your child’s growth velocity, and growth velocity is often more informative than any single blood test I could order that day.
And what a chart cannot do
A growth chart cannot tell you why. It can tell you that: that something has changed, that the trajectory is not what the family’s genetics predicted, that this child is not following their own established line. The why comes from the history, the examination, sometimes an X-ray of the left hand for bone age, and sometimes blood tests.
Plenty of the time, the answer is that everything is normal and the chart simply needed someone to look at it properly. That is a real result, and a good day.
This article is general information, not medical advice about your child. If you are worried about your child’s growth, bring the chart and the old records to a paediatrician or paediatric endocrinologist and have them looked at together.