How to read your child’s growth percentiles

Growth charts cause more parental anxiety than almost anything else in the first two years, and nearly all of it comes from reading the number as a score. It is not one. A percentile is a position, in the same way that shoe size is a position.

What the number means

If your six-month-old is on the 40th percentile for weight, then out of 100 six-month-olds of the same sex, roughly 40 weigh less and 60 weigh more. That is the whole claim. It says nothing about health, feeding or whether anything needs to change.

What each percentile means

Which lines you see printed depends on the chart. The WHO standards used under two mark the 3rd, 15th, 50th, 85th and 97th; CDC charts mark the 5th, 10th, 25th, 50th, 75th, 90th and 95th. That is why a child described as “on the 25th” at one appointment and “just under the 15th” at another may not have moved at all. The reading is the same in every case: the percentile is the share of children who measure less.

What the 3rd percentile means

3 children in 100 measure less; 97 measure more. It is the lowest line printed on the WHO chart, which makes it feel like an alarm, and for most children on it there is nothing wrong — someone has to be the smallest 3%. What earns attention is arriving there from a higher curve rather than sitting there steadily.

What the 10th percentile means

10 in 100 measure less. A CDC line rather than a WHO one, so under-twos are more often described as being between the 3rd and the 15th. Comfortably within normal.

What the 15th percentile means

15 in 100 measure less; 85 measure more. A WHO line, so it is one of the five an under-two is usually described against, and close enough to the CDC’s 10th that the same child can be called the 10th at one appointment and the 15th at the next without having moved.

What the 25th percentile means

25 children in 100 of the same age and sex measure less; 75 measure more. This is the one parents look up most, usually after being told it at a check-up and hearing it as a mark out of 100. It is not one. A child can sit on the 25th percentile their entire childhood and be exactly as healthy as one on the 75th.

What the 50th percentile means

The middle of the distribution: half measure less, half measure more. The most common misreading on the whole chart is treating it as the target. It is the average, and by definition half of all healthy children are below it.

What the 75th percentile means

75 in 100 measure less; 25 measure more. Normal, and no more informative than the 25th. A child here is not ahead of anything.

What the 85th percentile means

85 in 100 measure less. The WHO counterpart of the CDC’s 90th, which is the other half of the same confusion: one chart calls a child the 85th and the other the 90th, and neither has said anything different about the child.

What the 90th percentile means

90 in 100 measure less. Another CDC line — the nearest WHO equivalent is the 85th. Within normal, and worth reading alongside length: a baby on the 90th for both weight and length is simply a large baby, which is different from the 90th for weight against the 25th for length.

What the 97th percentile means

97 children in 100 measure less; 3 measure more. The top printed line on the WHO chart, and the mirror image of the 3rd — steady is reassuring, a climb across two lines to get there is the part a clinician would want to see.

Below the 3rd or above the 97th, a measurement is off the printed lines rather than off the scale of normal. It means a clinician looks at the whole picture — length, head circumference, feeding, the shape of the curve over months — instead of the single figure.

Which chart, and why

The WHO growth standards are built from children raised in conditions considered optimal — breastfed, non-smoking households, good healthcare — so they describe how children should grow rather than how a particular population happened to grow. That is why most health systems use them from birth to age two, then switch to CDC charts from two to five.

The practical consequence: do not compare a percentile from one chart to a percentile from another. The same measurement can land differently.

What clinicians are actually watching

Not the number — the line. A child establishes their own curve over the first months and is expected to travel roughly parallel to the printed lines after that.

• Crossing two or more major lines in either direction is the usual trigger for a closer look.
• A flat stretch where weight stops increasing over several weeks matters more than any single low reading — the expected weight gain by age is what tells you whether a stretch is genuinely flat.
• Head circumference is tracked separately, and a sudden change in its trajectory is followed up independently of weight and length.
• Some crossing is normal early on. Birth weight reflects the pregnancy; many babies shift up or down a band in the first months as they settle onto their own genetic curve.

Measuring well enough to be useful

• Weigh at a similar time of day, ideally before a feed, with the same amount of clothing.
• Length under two is measured lying down; height from two is measured standing. They differ, so the switch produces a small step in the chart.
• Home scales and clinic scales rarely agree. Comparing readings from one source beats mixing them.
• Weekly weighing is almost never necessary and reliably produces alarming-looking noise.

How Pixy plots it

Pixy records weight, length and head circumference and plots them against WHO percentiles, with milestones marked along the way. The point of keeping it in the app rather than on the back of the red book is that the trend is visible at a glance — which, as above, is the part that actually carries information.

When to raise it

Ask your paediatrician or health visitor if your child crosses two major lines, if weight plateaus, if head circumference changes trajectory, or if growth changes alongside feeding difficulty, vomiting or lethargy. A growth chart is a screening tool — it points at questions, it does not answer them.