Growth charts and what centiles mean

Few numbers cause as much parental anxiety as a centile, and few are as routinely misunderstood.

What a centile is

A position within a reference population.

A baby on the ninth centile for weight is heavier than nine per cent of babies of the same age and sex.

Half of all healthy babies are below the fiftieth centile by definition.

A low centile is not a low grade; a high centile is not an achievement.

Most charts in current use are based on the World Health Organization growth standards, derived from healthy breastfed infants in optimal conditions across several countries, and describe how children should grow rather than how a given population happens to grow.

What matters clinically

The trajectory rather than the position.

A baby tracking steadily along the second centile is usually a small baby growing well.

A baby dropping across two or more centile lines is a different matter and warrants assessment.

Which is why single measurements mean little and why repeated weighing at very short intervals is unhelpful and generates anxiety — guidance in several countries specifies minimum intervals between weighings for exactly this reason.

Head circumference and length matter alongside weight, and a discrepancy between them is more informative than any one.

The early weeks

Where most concern arises.

Weight loss after birth is expected, commonly up to around ten per cent of birth weight.

Loss beyond this, or continuing loss after day four or five, requires a feeding assessment.

Birth weight is typically regained by around two weeks.

After that, gain is rapid in the early months and slows progressively — a baby who gained two hundred grams a week at six weeks will not still be doing so at six months, and this deceleration is normal rather than faltering.

Breastfed and formula-fed patterns

Genuinely different.

Breastfed babies typically gain faster in the first months and more slowly from around three to six months compared with formula-fed babies.

Charts based on breastfed infants reflect this, and using an inappropriate reference can make a normally growing breastfed baby appear to be faltering — or a formula-fed baby appear to be gaining excessively.

Which is one reason charts changed, and why the chart used should be the current standard one.

Faltering growth

The term now used rather than failure to thrive.

It is defined by the pattern of centile crossing rather than by position, with specific thresholds depending on the starting centile.

Causes divide into inadequate intake, inadequate absorption, and increased requirements.

Inadequate intake is by far the most common — feeding difficulty, milk transfer problems, insufficient volumes, oral motor difficulty, or reduced appetite from illness.

Investigation is guided by the history and examination rather than by a battery of tests, and a feeding assessment by someone skilled is frequently the most useful single step.

Rapid weight gain

Less discussed and also relevant.

Very rapid weight gain in infancy has been associated in cohort studies with later obesity, particularly where feeding is not responsive.

Which is one argument for responsive bottle feeding — following the baby's cues rather than a volume target — and for not encouraging a baby to finish a bottle.

It is not a reason to restrict a baby's intake, which should never be done without medical advice.

Special situations

Where standard charts need adjusting.

Preterm infants are plotted using corrected age, generally until around one to two years, and specific preterm charts exist for the early period.

Some genetic conditions have condition-specific charts, since standard references misrepresent expected growth.

Parental size is relevant: small parents generally have small children, and mid-parental height is used to interpret a child's growth in older childhood.

How to think about it

Practical advice for parents.

Ask what the trend is rather than what the number is.

Do not compare with other babies, since a healthy population spans the whole chart.

Do not weigh at home frequently, since day-to-day variation from feeding, nappies and scales produces noise that looks like signal.

Use the same scales where possible.

Look at the baby as well as the chart — alertness, activity, wet nappies, developmental progress and contentment are the clinical picture that the number is meant to summarise.

And raise concerns rather than waiting for the next routine check, since feeding problems are considerably easier to address early.

General information only, not medical advice. Consult a health visitor or clinician about growth concerns rather than interpreting centiles alone.