What a Percentile Chart Shows

1. A percentile chart, also called a centile chart, plots a measurement against age.
2. The curved lines on the chart are percentile lines, and each one was drawn from measurements of a large sample of healthy children.
3. Health visitors in the United Kingdom use these charts to check that a baby is growing as expected, and the readings are recorded in the child's personal health record.
4. The three measurements normally monitored are body mass, length or height, and head circumference.
5. Boys and girls grow at different rates, so separate charts are used for each.

Reading a Percentile

1. A percentile tells you the percentage of the population that falls below a given value.
2. A baby on the 50th percentile for mass is exactly average, because half of babies of that age have a smaller mass and half have a greater mass.
3. A baby on the 91st percentile is heavier than 91 per cent of babies of the same age and sex, so only 9 per cent are heavier.
4. A baby on the 9th percentile is heavier than only 9 per cent, so 91 per cent are heavier.
5. The 50th percentile is the median, and the charts used in the United Kingdom usually print the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th and 99.6th lines.

Using the Chart

1. Find the child's age on the horizontal axis and read straight up.
2. Find the measurement on the vertical axis and read straight across.
3. Mark the point where the two lines cross, then read off the nearest percentile line.
4. If the point lies between two printed lines, state that the child is between those two percentiles rather than guessing an exact value.

Working Through an Example

1. A boy is 9 months old and has a mass of 9.6 kg, and his point sits on the 75th percentile line.

2. This means 75 per cent of 9 month old boys have a mass below 9.6 kg, so 25 per cent are heavier.

3. At birth the same boy had a mass of 3.2 kg, so his gain in mass can be calculated.

   $$percentage\; increase=\frac{9.6-3.2}{3.2}\times 100=200\%$$

4. His mass has therefore tripled in 9 months, which is typical for a healthy infant in the first year.

Following a Percentile Line

1. A healthy child normally grows along or close to the same percentile line over time.
2. Staying on the 9th percentile is not a problem in itself, because it simply means the child is small and growing steadily.
3. What matters is the pattern over several measurements, not a single reading.
4. This is why a health visitor plots repeated measurements rather than judging a child on one visit.

When a Chart Raises Concern

1. Crossing two or more percentile lines downwards may indicate a feeding problem, an illness or poor nutrition.
2. Crossing two or more lines upwards may indicate excessive weight gain and a raised risk of obesity.
3. A measurement below the 0.4th percentile or above the 99.6th percentile is followed up as a matter of routine.
4. An unusually large head circumference, or a rapid increase in it, can be an early sign of fluid building up in the skull.
5. A chart flags a child for further investigation, and it does not on its own diagnose anything.

Why Percentiles Are Used Rather Than Averages

1. A single average value hides the natural range of sizes in a healthy population.
2. Percentiles show the whole distribution, so a measurement can be judged against the spread and not just against the mean.
3. They also allow a fair comparison between children of different ages and sexes, because each chart is drawn for one group.