The growth chart is one of the most common sources of new-parent worry, and almost always for the wrong reason. Parents read it like a school report — higher is better, 50th is ideal, anything in single digits is bad. None of that is what the chart says. Once you know what the lines actually mean, the chart goes from a stress trigger to a useful tool.
Healthbooq lets you log weight and length over time and see the trajectory plotted out — far more useful than any single number on any single day.
What Centile Lines Mean
A growth chart shows a population distribution at each age — for weight, length, or head circumference. The centile lines mark out the spread:
- 50th centile: the middle of the population. Half above, half below.
- 9th centile: 9% of babies that age weigh the same or less; 91% weigh more.
- 91st centile: 91% weigh the same or less; 9% weigh more.
- 2nd / 98th: the edges of the standard normal range.
- 0.4th / 99.6th: the lowest / highest lines on the UK-WHO chart.
Every centile inside the printed range is a normal centile. A baby on the 5th centile is not unhealthier than a baby on the 95th — they are just smaller. There is no "good" centile to be on.
What does matter is the direction of the line your baby is drawing over time. A baby tracking steadily along the 9th centile from a few weeks old is growing well. A baby who started at the 75th and has dropped to the 25th by four months has crossed two centile lines, and that pattern deserves a conversation with a health visitor — even if 25th still sounds reassuring.
Which Chart: UK-WHO and CDC
In the UK, growth is plotted on the UK-WHO charts — based on the WHO Multicentre Growth Reference Study, which measured breastfed babies in six countries (Brazil, Ghana, India, Norway, Oman, USA) raised in optimal feeding and health conditions. These charts describe how babies grow when well-nourished, not how the average baby in any one country happens to grow.
In the US, the AAP recommends the WHO charts for under 2s and the CDC charts from 2 onwards. The CDC reference is a US population sample largely fed formula in the 1970s–90s, so plotted weight tends to look slightly higher on CDC than on WHO at the same age.
Two practical points: don't compare your baby's UK-WHO position to a friend's CDC position — they are different reference populations. And whichever chart your clinician uses, the principle is the same: the trajectory matters more than the point.
Weight: What's Normal
Birth weight is a baseline, not a starting line. Some babies are born big and find a slower growth track; some are born small and crossing upward; some hold their birth centile. Genetic regression to the mean — settling toward the centile their genes actually code for — happens in the first 4–6 weeks and can look like centile crossing without being a problem.
After the initial newborn weight loss (up to 10% of birth weight in the first 3–4 days, recovered by day 10–14), the rough weight-gain pattern is:
- 0–3 months: 150–200 g/week (around 25–30 g/day)
- 3–6 months: 100–130 g/week
- 6–12 months: 70–90 g/week
- 12–24 months: roughly 2–3 kg over the whole year
Breastfed babies tend to gain more variably than formula-fed — a bumper week, a slower week, evening out over a month. Formula-fed gain is usually steadier. Both are fine. Resist the urge to weigh weekly at home; once or twice a month at the health visitor clinic is plenty unless there is a specific concern.
Premature Babies: Use Corrected Age
A baby born at 32 weeks (eight weeks early) plotted on a chronological-age chart will look severely small. That's because the chart assumes 40-week gestation. Plot on corrected age — age since the original due date — until at least 12 months, often 24.
UK-WHO charts have a specific preterm section for the first few weeks; after that you adjust on the standard chart. Catch-up growth, where a preterm baby crosses centiles upward, is expected and welcome.
Head Circumference
Head circumference goes on its own chart. It is a measure of brain growth.
- A consistent centile (whatever centile) is reassuring
- Rapid upward crossing — particularly with a tense fontanelle, vomiting, or developmental concern — needs prompt assessment for hydrocephalus
- Falling away from the weight and length centiles needs a look
- Persistently above the 99.6th (macrocephaly) is often familial — measure a parent's head — but warrants a check
- Persistently below the 0.4th (microcephaly) needs assessment
A single head measurement isn't diagnostic. The trajectory is what your clinician reads.
When to Bring It Up
Worth raising with a health visitor or GP:
- Weight dropping two or more centile lines (the major lines: 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th, 99.6th)
- Birth weight not regained by day 14
- Visible weight loss after the first fortnight
- Head circumference growing significantly faster or slower than weight and length
- Persistently below the 0.4th centile
- Length falling off its line
- Reduced feeding, fewer wet nappies, or a generally less alert baby alongside the chart change
What this is not: a reason to panic. Most faltering growth in the UK is a feeding issue (latch, supply, technique, occasionally formula intolerance) that responds to support. Early conversation makes early fixes possible.
Key Takeaways
The centile your baby sits on is not a score. A baby on the 5th centile is healthy if they are tracking the 5th; one on the 95th is healthy if they are tracking the 95th. What matters is the direction of the line. A drop of two centile lines or more in weight — particularly if length is keeping up — warrants a feeding review. Expect roughly 150–200 g/week in the first 3 months, 100–130 g/week from 3–6 months, and 70–90 g/week from 6–12 months. Premature babies are plotted on corrected age. UK uses UK-WHO charts; US uses WHO under 2 and CDC after.
