What are growth charts and centiles actually telling you?

Where a child sits compared to a reference population — and nothing else. A centile is a ranking, not a grade, and the single most common misreading is treating a low one as a problem and a high one as an achievement.

How to read it. A child on the 25th centile for weight is heavier than 25% of children of the same age and sex. Someone has to be on the 9th centile — by definition around 9% of healthy children are, and they are not failing at anything.

Why the trajectory matters far more than the number. Children broadly track along a centile line. What clinicians look for is change: a child who has followed the 50th and drops to the 9th over several months is of interest precisely because the pattern changed, even though 9 is a perfectly ordinary number. A child who has always been on the 2nd and continues along it is usually doing exactly what they should.

What the UK charts are based on. The WHO growth standard for babies, built from healthy breastfed infants across several countries growing in favourable conditions. That word matters: it is a standard describing how children should grow, not merely a reference describing how a sample did grow.

Specific things that cause unnecessary alarm:

Newborn weight loss. A drop in the first days is expected; it is the percentage and the recovery that matter.

Formula-fed babies may track differently from the standard.

Preterm babies need corrected age plotting, or the picture is meaningless.

Head circumference is measured because change in its trajectory is clinically meaningful, and it varies genuinely between children.

Single measurements are noisy — a squirming toddler, different scales, clothing.

BMI centiles for older children are read against age and sex, not adult thresholds.

Parental size is relevant context that a chart cannot know.

General information, not medical advice.