Baby percentile chart explained: what the centile lines really say

Baby percentile chart: what the number means

A baby percentile chart compares one measurement with children of the same age and sex in a reference group. In the UK the same idea is usually called a centile, and the lines in your baby's red book are centile lines. Percentile and centile mean the same thing.

The number is a rank, nothing more. The Royal College of Paediatrics and Child Health (RCPCH) explains it like this: if your child is on the 25th centile, and you lined up 100 children of the same age and sex, your child would probably be number 25. The 50th centile is the middle of the group. It tells you where a measurement sits in comparison, not whether it is good or bad.

Which growth chart is used: UK-WHO, WHO and CDC

Every growth chart rests on a reference. Which one you see depends on where you live.

In the UK: the UK-WHO charts in the red book

In the UK, your baby's growth is recorded on centile charts in the Personal Child Health Record, the red book. These are the UK-WHO growth charts, published by the RCPCH in 2009. For children under four they are built on the WHO Child Growth Standards. Because the WHO data do not include babies born early, the birth section of the UK-WHO charts uses British data from 1990.

The UK-WHO charts have nine centile lines: the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th and 99.6th. Between birth and two weeks there are no centile lines at all. The RCPCH explains that most babies lose some weight after birth and regain it, and that patterns vary too widely in those days for a chart to show.

In the US: WHO charts up to age two

For readers in the United States: the CDC recommends the WHO growth standard charts for children from birth to two years, and the CDC growth charts from two years on.

The WHO Child Growth Standards

The WHO standards cover children from birth to five years and come from the WHO Multicentre Growth Reference Study. The WHO publishes the charts and tables openly, separated by measurement, by sex and by age range.

How to read a WHO growth chart

A growth chart has two axes. Along the bottom is age, up the side is the measurement: weight, length or height, or head circumference. Each measurement has its own chart, and boys and girls have separate ones.

The curved lines join the same centile across all ages. The line in the middle is the 50th. A dot on the chart shows where one measurement sat in the reference on one particular day.

Length and height: the switch at two years

For babies and children under two, length is measured lying down rather than height standing up, as the RCPCH notes. The WHO keeps two separate charts for this: length-for-age from birth to two years, and height-for-age from two to five years. If you see both on one chart, the lines have a small step at 24 months. It comes from the change in how children are measured, not from a change in the child.

One measurement is a snapshot

Centiles describe; they do not judge. A single dot shows one day, not a pattern.

The NHS puts it plainly for parents: a baby's weight and height may not follow a centile line exactly. Measurements may go up or down by one centile line; crossing two lines is less common. It is also normal for a baby to be on different centiles for weight and length, though the two are usually fairly similar.

The RCPCH adds two things worth knowing. Every child is different, so no two filled-in charts look the same, and even twins can grow differently. And weighing very often is not always helpful. A baby weighed just after a big feed one week and before a feed the next can look as if nothing has changed.

Why your baby's own line says more than one number

A growth chart only takes shape with several dots. Each new measurement shows how your baby is growing over weeks and months, compared with themselves. That line tells you more than whether today's weight sat on the 40th or the 45th centile.

How a pattern should be read is for your health visitor, GP or paediatrician. They know how the measurement was taken, the history, and above all your baby. The RCPCH also recommends that plotting in the red book is left to trained professionals, because it takes practice to plot accurately.

The regular reviews are a good place to ask about the chart. You can find the schedule under NHS baby reviews and some calm ideas for the appointment under preparing for checkups. If you are worried, talk to your health visitor or GP, whichever centile a measurement is on.

What this page deliberately does not do

There is no percentile calculator here and no sorting into "fine" or "unusual". A number without context can unsettle more than it helps, and whether a measurement or a pattern needs a closer look can only be judged by someone who sees your baby. What this page tries to do is help you understand what is on the chart, so you can go into the appointment with good questions.

The growth chart in Heia

In Heia you can add weight, length and head circumference, for example straight after a weigh-in. Heia plots your measurements on the WHO Child Growth Standards: a shaded WHO reference range from P3 to P97 and a dashed line for the WHO middle. The growth chart is part of the free everyday app.

• Tap any dot to see the date, age, value and percentile.
• Its position is described in plain words, such as "within the WHO reference range, about in the middle". Heia describes; it does not judge.
• For a baby born more than three weeks early, Heia uses the adjusted age up to 24 months, if you have entered a due date.
• Measurements before the due date appear in the list but not on the chart, because the WHO chart starts at the due date. The WHO reference ends at five years; later measurements stay in the list without a percentile.

Heia's chart is not a copy of the red book. The red book uses the UK-WHO charts with nine lines from the 0.4th to the 99.6th; Heia shows the WHO standards with a shaded range from P3 to P97. So the picture can look a little different. For decisions, the chart your health professional keeps is the one that counts. Heia is your own record in between. If you track together, everyone in your Household sees the same chart; see shared baby tracker.

Frequently asked questions