Growth is among the most objective and measurable aspects of a child's development. It is also among the most anxiety-provoking, because weight in particular is visible and quantifiable in a way that other aspects of development are not. Making sense of growth charts – understanding what they show, how to read them, and what actually warrants concern – is one of the most practically useful pieces of knowledge a parent can have in the first three years.
Healthbooq covers infant and child health, including growth monitoring through the early years.
The UK-WHO Growth Charts
The UK has used the WHO Child Growth Standards since 2009, adapted with some UK-specific features. These charts were derived from data collected in six countries (Brazil, Ghana, India, Norway, Oman, and the United States) on children who were breastfed and raised in conditions optimised for growth. The population was specifically selected to represent how children grow under ideal conditions, not simply how they grow on average.
Charts track weight (from birth), length (lying, until approximately 2 years) or height (standing, from approximately 2 years), and head circumference. All three measurements have their own chart lines.
What Centile Lines Mean
A centile line represents the percentage of a healthy reference population that falls below that value. The 50th centile is the median: half of the reference population falls above and half below. The 91st centile means 91% of the population falls below that value (and 9% above). The 2nd centile means only 2% of the population falls below that value.
Being on the 2nd centile is not the same as being small for an abnormal reason. A child on the 2nd centile who follows that centile consistently, is active, feeding normally, and developmentally well, is simply smaller than most children their age – which, by definition, some children must be.
Expected Growth Rates in the First Three Years
The rate of growth is not constant – it is fastest in the first months of life and decelerates progressively.
0-3 months: approximately 150-200g per week (around 1kg per month). This is the fastest relative growth rate of a human life.
3-6 months: approximately 100-150g per week.
6-12 months: approximately 70-90g per week.
1-2 years: approximately 2-3kg across the entire year (far slower than in infancy).
2-3 years: approximately 2kg across the year.
Length/height growth follows a similar deceleration: approximately 25cm in the first year, then 12-13cm in the second year, and 8-9cm in the third year.
Head Circumference
Head circumference growth reflects brain growth and is particularly significant in the first two years when brain volume increases most rapidly. The brain roughly doubles in size in the first year and reaches approximately 80% of its adult volume by age 3. Measuring head circumference accurately requires a non-stretchable tape measure at the widest point of the skull.
Normal head circumference at birth is approximately 34-36cm; by 1 year, approximately 44-47cm; by 2 years, approximately 48-50cm. Crossing upward centile lines (macrocephaly) may indicate hydrocephalus; crossing downward significantly (microcephaly) may indicate problems with brain growth.
What Warrants Investigation
Crossing two or more centile lines downward in weight, height, or head circumference over a monitoring period is the threshold used in UK clinical practice to trigger assessment. Some centile crossing is expected and normal – a baby born on the 91st centile who settles on the 50th centile as their genetic potential becomes expressed is not in trouble.
Faltering growth (formerly termed failure to thrive) describes persistent inadequate weight gain. The causes range from feeding difficulties (the commonest UK cause) to malabsorption (coeliac disease, food allergy, cystic fibrosis) to endocrine disorders.
Early referral to a health visitor or GP for growth concerns allows intervention before deficits become significant.
Key Takeaways
Growth monitoring in the first three years uses UK-WHO growth charts to track weight, length/height, and head circumference. Centile lines represent the distribution of measurements in a healthy population – being on the 9th centile is not abnormal, it means the child is smaller than 91% of children their age. Following a consistent centile trajectory is more important than the specific centile. Normal weight gain slows substantially in the first year: rapid in the first 3 months (150-200g/week), progressively slower through the second half of the first year, and then approximately 2-3kg across the whole of the second year. Head circumference growth reflects brain growth and is particularly important in the first two years.
