How to read a baby growth percentile
When our first arrived she sat stubbornly on the 9th percentile for weight, and I remember refreshing charts at two in the morning convinced we were failing her. The health visitor said something I have repeated to friends ever since: a percentile is a position in a queue, not a grade. The 9th percentile is a perfectly healthy place to stand in that queue, as long as you keep standing roughly there.
The WHO Child Growth Standards describe how healthy, breastfed children grow when nothing gets in their way. They were built from 8,440 children across Brazil, Ghana, India, Norway, Oman and the United States in the Multicentre Growth Reference Study, and growth turned out to be strikingly similar across all six countries. That is why the same curves are used in the NHS red book, by the CDC for children under two, and by paediatric societies across Asia.
Boys and girls grow on different curves
Boys are a little heavier and longer than girls from birth, and the gap widens slightly over the first year. The WHO median birth weight is about 3.3 kg for boys and 3.2 kg for girls, and by twelve months the medians are roughly 9.6 kg and 8.9 kg. Plotting a girl on a boy chart would make her look smaller than she is, so always check the sex toggle first.
| Age | Boys median weight | Girls median weight | Boys median length | Girls median length |
|---|---|---|---|---|
| Birth | 3.3 kg | 3.2 kg | 49.9 cm | 49.1 cm |
| 3 months | 6.4 kg | 5.8 kg | 61.4 cm | 59.8 cm |
| 6 months | 7.9 kg | 7.3 kg | 67.6 cm | 65.7 cm |
| 12 months | 9.6 kg | 8.9 kg | 75.7 cm | 74.0 cm |
| 24 months | 12.2 kg | 11.5 kg | 87.1 cm | 85.7 cm |
Source: WHO Child Growth Standards, weight-for-age and length-for-age median (50th percentile) values.
What the Asian ancestry view does, and does not do
Babies of South Asian and East Asian ancestry are on average born 200 to 300 g lighter than white European babies, a difference seen consistently in the INTERGROWTH-21st project (Villar et al, Lancet 2014) and in national birth statistics in the UK, US and Singapore. Because the WHO curves already include Indian children and set a standard for optimal growth, the official advice from WHO, the NHS and the CDC is to use them unchanged for every ethnicity.
Some paediatricians in India and Singapore nonetheless interpret a low weight percentile more gently in an Asian baby who is feeding well and tracking a steady line. The ancestry toggle shows that second reading alongside the official WHO percentile, so you can see both and talk it through with your doctor. It does not replace the WHO number, and a falling curve deserves attention whatever your background.
When a percentile is worth a conversation
- Crossing two major lines downward (for example 50th to below 15th) over a few weeks, especially with fewer wet nappies or a sleepy, hard to wake baby.
- Below the 2nd or above the 98th percentile for weight, or a large mismatch between weight and length percentiles.
- Head circumference crossing lines quickly in either direction.
- Slow gain after a fast start is common at 3 to 4 months in breastfed babies and is usually normal; the RCPCH UK-WHO chart guidance explains why.
Frequently asked questions
- What is a baby weight percentile?
- A percentile tells you where your baby sits compared with healthy, breastfed babies of the same age and sex in the WHO Child Growth Standards. The 40th percentile for weight means 40 out of 100 babies weigh less and 60 weigh more. Any percentile between the 3rd and the 97th is considered within the normal range.
- Are the percentiles different for boys and girls?
- Yes. WHO publishes separate tables for boys and girls because boys are on average slightly heavier and longer from birth. This calculator switches curves the moment you change the sex toggle, so a 6 kg girl and a 6 kg boy at three months will show different percentiles.
- Do WHO growth charts work for Asian babies?
- Yes. The WHO standards were built from children in Brazil, Ghana, India, Norway, Oman and the USA who were breastfed and raised in healthy conditions, and growth was remarkably similar across all six sites. That is why the NHS, the CDC and paediatric bodies in India and Singapore all use them. Asian babies are, on average, born slightly lighter, so this tool also offers an ancestry view that re-expresses the result against a lower reference median, but the WHO percentile itself does not change.
- What percentile is a concern for a baby?
- Paediatricians pay more attention to the direction of travel than to any single number. A baby who has always tracked the 10th percentile is usually fine. A baby who drops across two major percentile lines, for example from the 50th to below the 15th, or who sits below the 2nd percentile or above the 98th, should be reviewed by a health visitor, GP or paediatrician.
- How much weight should a baby gain each month?
- As a rough guide from WHO data, babies gain about 150 to 200 g a week in the first three months, 100 to 150 g a week from three to six months, and 70 to 90 g a week from six to twelve months. Most babies double their birth weight by around five months and triple it by around a year, then slow considerably in the second year.
- Why does the NHS use WHO charts but the US uses CDC charts after age 2?
- The CDC recommends WHO charts for children under 2 because they describe how healthy breastfed babies should grow. From age 2 the CDC switches to its own 2000 reference, which describes how US children actually grew. The UK-WHO charts in the NHS red book use WHO data from 2 weeks to 4 years and UK 1990 data after that.
This calculator is for general information and is not medical advice. Home scales and tape measures are less accurate than clinic equipment, and premature babies need corrected age. If you are worried about your baby's growth or feeding, speak to your health visitor, GP or paediatrician.