WHO Growth Charts for Babies: Percentiles and Centiles Explained

Every parent wants to know: is my baby growing normally? The World Health Organization (WHO) has spent decades collecting growth data from healthy children worldwide. The result: the most reliable reference charts available — and a standard many pediatricians use.

Here's what you need to know about interpreting these charts, with the actual numbers.

What Are WHO Growth Charts?

WHO growth charts track six measurements: weight-for-age, height-for-age, weight-for-height, head circumference, arm circumference, and BMI. For most parents, the first three are most relevant.

Charts are split by sex — boys and girls grow at different rates, especially in the first two years.

You can work out your own baby's centile here — weight, length or head circumference, with corrected age for premature babies.

Results are expressed as percentiles — called centiles in the UK, Ireland and Australia. The two words mean exactly the same thing: if your health visitor says your baby is "on the 25th centile" and an American article says "25th percentile", they are describing an identical measurement. A baby on the 50th centile is around the middle of the reference group. The 25th centile means they're larger than 25% of babies their age. Neither high nor low centiles are automatically good or bad — the trend matters more than a single measurement.

Weight-for-Age: Boys (50th Percentile)

Weight-for-Age: Girls (50th Percentile)

Height-for-Age: Boys (50th Percentile)

Height-for-Age: Girls (50th Percentile)

If You're in the UK: Centile Charts and the Red Book

British parents meet these charts in a different place and with different numbers, which is why the same measurement can look unfamiliar when you compare notes across countries.

In the UK, growth is plotted on UK-WHO growth charts, published by the Royal College of Paediatrics and Child Health. They use WHO data from two weeks to four years, joined to UK reference data for the newborn period. The chart itself lives in the red book — the Personal Child Health Record you're given after birth — and it's normally filled in by your health visitor.

The visible difference is the spacing of the lines. UK-WHO charts print nine centile lines:

0.4th · 2nd · 9th · 25th · 50th · 75th · 91st · 98th · 99.6th

American charts usually print the 3rd, 5th, 10th, 25th, 50th, 75th, 90th, 95th and 97th instead. Nothing about your baby changes — only which lines happen to be drawn. A baby measured at the 9th centile in London and the 10th percentile in Chicago is the same size.

Two phrases British parents hear a lot, and what they actually mean:

• "Dropped a centile" — the plotted point has crossed below one printed line. Crossing one line is common and often nothing; crossing two lines downward is the point at which a health visitor or GP usually wants a closer look.
• "On the 25th centile" — a quarter of babies of the same age and sex are smaller, three quarters are larger. It is a position, not a grade.

Sunny Seed works centiles out on your phone, with no network call, using the LMS formula the standard itself defines. Two things are worth knowing about the number it returns. It carries a condensed version of the WHO tables — anchor ages, with the values between them interpolated — so a figure for a 26- or 40-month-old is very close to the published one rather than lifted straight from it. And a UK-WHO chart is not purely WHO for the newborn weeks: that stretch is joined to UK reference data, which the app does not hold. The app is not affiliated with the NHS or the RCPCH and does not replace the red book; it keeps a running record with you between appointments, when the book is at home in a drawer.

Month-by-month pages pull these figures together with sleep norms, milestones and the vaccine doses due at that age: newborn · 2 months · 6 months · 12 months · 24 months.

How to Read the Chart Correctly

Single measurements mean little

A snapshot tells you where your baby is today. What matters is whether they're following their own curve. A baby consistently at the 10th percentile — the 9th centile line on a UK chart — may be perfectly healthy, they're just small. Concern arises when a baby drops two or more centile bands between visits.

Weight gain by phase

Babies grow fastest in the first months:

• 0–3 months: ~200 g/week (about 30 g/day)
• 3–6 months: ~130 g/week
• 6–12 months: ~70–90 g/week
• After 12 months: ~50 g/week

If weight gain slows noticeably, talk to your pediatrician.

Head circumference matters too

Head circumference reflects brain growth. Normal range at birth: 33–37 cm. It grows about 2 cm per month in the first three months, then slows. Your pediatrician measures this at every visit.

Premature Babies: Corrected Age

For babies born before 37 weeks, pediatricians often use corrected age (chronological age minus weeks premature) when interpreting growth charts, especially in the first 2 years and sometimes longer for extreme prematurity. Follow your pediatrician’s guidance.

A baby born 8 weeks early who is 4 months old should be plotted at 2 months on the growth chart.

When to See Your Pediatrician

Schedule a visit (don't wait for the next routine check) if:

• Your baby drops 2+ percentile bands between measurements
• Weight gain stops for 2+ weeks under 6 months, or 4+ weeks after 6 months
• Head circumference is below P3 or above P97
• You're worried — trust your gut

For month-by-month reference values, see the baby weight chart by month.

How to Track Accurately

Weigh babies naked, on the same scale. Measure length lying down (under 2 years) — standing measurements are up to 2 cm shorter. Take measurements at the same time of day.

A single pediatric visit gives you one data point. Tracking at home between visits gives you the trend — which is what matters.

The best approach: record every measurement with dates in a baby growth tracker, so you can see the curve building over time. When your pediatrician asks "how has weight gain been?", you'll have a real answer — not a guess.

For a calmer explanation of percentiles and growth trends, read baby growth chart explained.