How to Read WHO Growth Charts: Percentiles Explained for Parents

A percentile on a growth chart compares your baby with other babies of the same age and sex. If your baby's weight is on the 25th percentile, about 25 out of 100 babies of that age weigh less and about 75 weigh more. It is a position, not a score. A baby on the 9th percentile can be just as healthy as one on the 91st.

What doctors and health visitors watch is the line your baby follows over time, not a single dot.

Where the WHO charts come from

The WHO Child Growth Standards were published in 2006. They are based on the WHO Multicentre Growth Reference Study, which followed 8,440 healthy breastfed children in Brazil, Ghana, India, Norway, Oman, and the USA between 1997 and 2003. The families lived in supportive conditions and did not smoke.

That design matters. The charts describe how children grow when they are healthy and well fed, rather than the average of whatever population happened to be measured. They cover birth to 5 years.

In the US, the CDC recommends the WHO charts from birth to age 2, and its own charts after that. In the UK, the red book uses the UK-WHO charts, which are built on the same WHO data but drawn with more centile lines.

The three charts for babies

• Weight-for-age is the one checked most often in the first months.
• Length-for-age uses length lying down until about age 2.
• Head circumference-for-age is measured at birth and at routine checks.

Each one has separate versions for boys and girls, because they grow differently.

Reading the lines

Most WHO charts for parents show five curves: the 3rd, 15th, 50th, 85th, and 97th percentiles. The 50th is the median.

Some things to know when you look at your baby's dots:

1. Weight drops after birth. Almost all newborns lose some weight in the first week. Most are back to birth weight by about 2 weeks, and nearly all by 3 weeks.
2. Small moves are normal. The NHS notes that measurements can move up or down by one centile line.
3. Crossing two lines is worth asking about. It is less common, and a good reason to talk to your health visitor or pediatrician. It doesn't automatically mean something is wrong.
4. One measurement can be off. Different scales, a full diaper, or a wriggly baby on the length board all add noise. Several measurements tell you more than one.

Keeping your own record

Clinic visits are weeks apart, and the paper book is not always to hand. Writing each measurement down with its date makes it easy to answer "how has the weight been since the last visit?"

BabyTrack+ has a growth tab for exactly this. You enter weight, length, or head circumference, and it plots the measurement on the WHO curves for your baby's sex from birth to 24 months, with the percentile next to it. You can share the measurement history as text, for example in a message to your pediatrician. Units can be metric or imperial.

The app only does the arithmetic against WHO reference data. It doesn't diagnose anything, and if the numbers worry you, your pediatrician or health visitor is the person to ask.

For what to log alongside growth in the early weeks, see how often a newborn should feed.