Plot your baby’s weight, length and head size on WHO centile charts.
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What centiles mean
A baby on the 25th centile for weight is heavier than 25 out of 100 healthy babies of the same age and sex, and lighter than 75. There’s no “right” centile: what matters is that your baby grows steadily, roughly following a line.
When to get advice
NICE guidance says health professionals should be concerned about faltering growth if a baby’s weight drops:
| Birth weight centile | Concern if weight falls across |
|---|---|
| Below the 9th | 1 or more centile spaces |
| 9th to 91st | 2 or more centile spaces |
| Above the 91st | 3 or more centile spaces |
| Any | Current weight below the 2nd centile |
Most babies lose some weight in the first few days and are back to their birth weight by 3 weeks. The tool flags these patterns, but your health visitor or GP should always check.
Premature babies
For babies born before 37 weeks, centiles are worked out from their corrected age: until 1 year for babies born at 32 to 36 weeks, and until 2 years for babies born before 32 weeks.
Frequently asked questions
Are these the same charts as the red book?
The UK-WHO charts in the red book use the same WHO standards from 2 weeks of age, with UK data for birth, so birth centiles can differ slightly.
Can I enter weight in grams?
Enter kilograms, such as 4.25. If you type a number over 30, the tool assumes grams and converts it.
Is my data private?
Yes. It’s saved in this browser only and never uploaded.
Sources
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