Growth charts are one of the most powerful tools in paediatric medicine — and one of the most misunderstood by parents. This guide explains what the lines on the chart actually mean, what constitutes a genuine concern versus normal variation, and how growth changes in the first three years. For a comprehensive overview, see our complete guide to child health.
How Growth Charts Work
The UK uses WHO growth charts for children from birth to age 4. These charts were developed from data on healthy children worldwide who were breastfed for at least 12 months, which is why they are considered the reference standard for normal growth.
The chart shows centile curves: the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th, and 99.6th centiles for weight and height.
- A child on the 50th centile for weight is exactly average: half of children weigh more, half weigh less.
- A child on the 9th centile is lighter than 91% of children the same age — but this is entirely normal.
- A child on the 91st centile is heavier than 91% of children — also normal.
Every centile position from the 0.4th to the 99.6th is considered within the normal range. There is no "ideal" centile.
What Is Normal Growth in the First Three Years?
0–3 months: Fastest growth of childhood. Average weight gain is 150–200 g per week. Length increases rapidly.
3–6 months: Still fast, but slowing slightly. Weight gain averages 100–150 g per week.
6–12 months: Slower. Around 70–90 g per week.
12–24 months: Growth slows markedly. Many toddlers gain only 1.5–2 kg in the entire second year. This is normal and coincides with a decrease in appetite that many parents mistake for a problem.
24–36 months: Continues at a relatively slow pace. Height gain begins to be more prominent than weight gain as body proportions change.
What to Watch For: When Measurement Patterns Matter
Crossing centile lines downward: A single lower measurement may reflect inaccurate measurement. A consistent downward trend across two or more centile lines (e.g., from 75th to below 25th) over several months suggests faltering growth, which needs investigation. Causes range from inadequate intake (feeding difficulties, low supply) to underlying medical conditions.
Crossing centile lines upward rapidly: Unexplained rapid weight gain may warrant review, particularly if the child's height centile isn't keeping pace.
Measurement not matching parental size: If both parents are tall and a child is tracking on the 2nd centile for height, this might warrant investigation for growth hormone deficiency or other causes of short stature. Conversely, a child of small parents on the 5th centile is almost certainly constitutionally small.
Head Circumference
Head circumference reflects brain growth and is measured at birth and at routine checks. Average at birth is 33–35 cm; it reaches about 46–47 cm by 12 months. Small deviations from average centile are usually constitutional. Persistent crossing of centile lines (upward or downward) warrants evaluation.
Common Misinterpretations
"My baby dropped from the 50th to the 25th centile." This may represent natural settling onto a genetically appropriate centile, particularly in the first weeks of life or around 6–9 months. One-to-one comparisons with previous measurements matter; the overall trend over several months matters more than any single shift.
"My toddler has barely gained any weight this month." In the second year, weight gain of a few hundred grams per month is entirely normal. If your toddler is active, alert, eating, and following their centile line, no concern is warranted.
"My baby is small — is something wrong?" In the context of a baby who is feeding well, alert, and developmentally on track, being on a low centile is almost certainly constitutional (genetic). The centile line matters; the line's position doesn't.
Practical Guidance
Bring your red book to every health visitor and GP appointment so weight and height can be plotted consistently. If you are concerned about your child's growth, discuss it with your health visitor or GP. A single reading is rarely informative; it's the trend over time that tells the story.
Key Takeaways
Growth centile charts show where a child sits in the normal distribution (all centiles 0.4-99.6 are normal); there is no ideal centile. Normal growth patterns: 0-3 months averages 150-200g/week; 3-6 months 100-150g/week; 6-12 months 70-90g/week; and second year weight gain slows markedly to 1.5-2kg total. Crossing two centile lines downward (faltering growth) or upward rapidly warrants investigation. Growth measurement trends over time matter more than single readings.
