Baby Growth Chart Percentiles Explained: Why the 25th Is Not a Score

The nurse plots a dot in your baby's book, says "25th percentile", and moves on. You spend the drive home wondering why your baby only got 25 out of 100 — and whether you should be doing something about it. The same dot gets plotted again at each of the scheduled baby health checks in Australia, and that repetition is the point.

Here's the reframe that takes the sting out: a percentile is a position, not a mark. It says where your baby sits compared with other babies of the same age and sex — nothing more. The 25th percentile simply means that out of 100 typical babies, about 25 are lighter and about 75 are heavier. A perfectly healthy baby can live their whole first year on the 10th percentile, and another on the 90th, and neither is "doing better" than the other.

What actually tells you something is the shape of the line over time — and that's the part a single dot can never show.

Is the 25th percentile good?

The 25th percentile is not a score, and on its own it is neither good nor bad. It means about a quarter of babies the same age and sex measure less than yours, and about three quarters measure more. A healthy baby can sit on the 5th or the 95th and stay there for their whole first year.

What clinicians actually watch is the shape of the line over successive visits — whether your baby keeps tracking along their own curve, not which curve it is. A baby who has been on the 25th since birth is a small baby growing well. A baby who was on the 75th and has slid to the 25th is the same dot today and a different conversation.

That's also why one weigh-in rarely settles the worry: the answer only appears once there are a few months of points on the same chart. The rest of this article is about reading that line — what the chart is, what the numbers mean, and when the trend is worth raising with your GP.

What a growth chart actually is

The charts in your baby's health record for 0–2 years are the WHO Child Growth Standards(opens in a new window) — curves built from the growth of healthy, breastfed babies raised in good conditions across six countries. All Australian states and territories use these WHO charts for children under two, and the Royal Children's Hospital Melbourne(opens in a new window) publishes the versions your child health nurse plots on.

WHO · Child Growth Standards

The WHO growth standards describe normal child growth from birth to 5 years under optimal environmental conditions, and can be applied to all children everywhere, regardless of ethnicity, socioeconomic status and type of feeding.

Three measurements go on the chart: weight, length (height once they're standing) and head circumference. Each gets its own curve, and each is read the same way — as a position among peers, not a grade.

What the percentile numbers actually mean

Notice what's missing from that table: any notion of pass or fail. As Raising Children Network(opens in a new window) puts it, a baby on the 5th percentile and a baby on the 90th are both within the typical range — the percentile just describes where in that range they sit.

Half of all perfectly healthy babies sit below the 50th percentile. That's not a problem — it's what "average" means.

The trend line is the whole story

A worked example. Two babies are both weighed at six months and both land on the 25th percentile — the identical dot on the chart.

• Baby A was born around the 25th percentile and has tracked along it at every check: 25th at birth, 25th at two months, 25th at six months. A steady line, parallel to the printed curves. This is textbook healthy growth for a baby who is simply built on the smaller side.
• Baby B was born on the 75th percentile and has slid — 75th at birth, 50th at three months, 25th now. Same dot today, very different story: this baby is crossing centile lines downwards, away from their own established curve.

Baby A's parents don't need to do anything. Baby B's parents should mention the pattern to their GP or child health nurse. The single measurement couldn't distinguish the two — only the trend could. That's why Pregnancy, Birth and Baby(opens in a new window) sums the whole topic up in one line: the important thing is that your baby continues to grow at a similar rate.

For the grams behind the dots, our guide to how much weight a newborn should gain each week gives the Australian figures by age and a worked 4-week average.

If your baby was born early: read the chart at their corrected age

There's one situation where the trend line is easy to misread, and it catches out a lot of families: a baby born preterm. Plot a preterm baby on the WHO 0–5 year charts using their birthday, and the line sits low from the very first dot — not because the baby is growing badly, but because they're being lined up against babies who got extra weeks of growing before they were born. In the first year or two that gap is big enough to make a thriving baby look small on paper. The fix isn't a different chart. It's a different age.

Corrected age is your baby's age now minus the number of weeks they arrived early, counted back from a 40-week due date. Pregnancy, Birth and Baby(opens in a new window) states it plainly — corrected age = chronological age minus how many weeks premature — and works it through: a baby who is 16 weeks old and was born 8 weeks premature has a corrected age of 8 weeks. On the same arithmetic, a one-year-old born three months early has a corrected age of nine months.

That's also why the exact gestation at birth matters so much when anyone reads a preterm percentile out loud. Four weeks either way moves the dot along the age axis, and on the steep part of the curve — the first few months — moving the dot sideways moves the percentile a long way. A percentile quoted without the correction isn't wrong so much as it's answering a different question.

This is standard practice here, not a workaround. The Royal Children's Hospital Melbourne(opens in a new window) teaches it with a worked case: a baby born at 32 weeks and measured at 10 months of age is assessed at 8 months corrected, and once her age is corrected her weight and length both track along the 3rd percentile rather than looking like a stall. The Sydney Children's Hospitals Network(opens in a new window) uses the same subtraction for development. And it runs longer than most people expect: Pregnancy, Birth and Baby says a baby's corrected age is useful until at least 2 to 3 years of age, so it isn't an adjustment you quietly drop after the newborn months.

Once you're reading the chart at the corrected age, everything else in this article applies unchanged: the percentile is still a position rather than a mark, and the shape of the line over successive visits is still the part worth watching. If the corrected-age line is drifting down through the printed curves, that's the same conversation with your GP or child health nurse as it would be for any other baby.

When to check in with your GP

Worth a conversation with your GP or child health nurse:

• Crossing centile lines — weight moving down (or sharply up) through two or more of the printed curves over successive checks
• Dropping off their own curve — a baby who tracked the 50th for months now trending well below it
• Measurements moving in opposite directions — for example, length climbing while weight falls away
• Your gut says something's off — feeding refusal, fewer wet nappies, unusual lethargy alongside slowing growth

None of these are diagnoses; they're patterns that deserve a professional's eyes. And a low-but-steady line isn't on the list — small and consistent is a build, not a warning.

What a head circumference percentile means

A head circumference percentile compares the distance around your baby's head with healthy babies of the same age and sex on the WHO charts. On the 90th, about 90 in 100 babies have a smaller head. Nurses want a head that grows steadily along its own curve, in step with weight and length.

Head size is read a little differently from weight. Weight and length are judged mostly on the trend, and a low but steady line is fine. For head circumference, a reading below the 3rd or above the 97th line can itself be a reason for a doctor to take a closer look, even when the line is steady.

So a head on the 90th percentile is inside the printed range, below the top line at the 97th. If it stays near the 90th and matches the rest of the body, it is simply a bigger head.

Why head size is measured at checks

Measuring head circumference is a quick, non-invasive way to track brain growth. The WA Child and Adolescent Health Service(opens in a new window) notes that the brain grows fastest in the last three months of pregnancy and the first two years of life. That is why the measurement matters most before your child turns two.

The Royal Children's Hospital Melbourne(opens in a new window) says babies under 1 should have their head measured at child health checks, along with weight and length. In WA, nurses measure it at the 8 week, 4 month and 12 month checks, and at any other visit where there is a concern.

The RCH measuring guide(opens in a new window) adds that problems with head growth usually have causes other than nutrition. A head line is not a feeding report in the way a weight line can be.

The middle line on the WHO head circumference charts(opens in a new window) is the 50th percentile. There is one chart for girls and one for boys, and boys' heads run a little bigger.

Healthy babies sit above and below these numbers. The percentile line tells you more than the centimetres.

How the nurse measures and plots it

The nurse wraps a flexible tape around the widest part of the head: just above the eyebrows, above the ears and around the bump at the back of the head. The tape is pulled gently to flatten the hair, and the result is written down to the nearest millimetre.

If a reading looks out of line with earlier ones, the nurse measures again. WA nurses take three measurements and record the largest.

The number goes on the WHO head circumference-for-age chart, which has separate versions for girls and boys from birth to 2 years. The RCH chart(opens in a new window) prints five curves: the 3rd, 15th, 50th, 85th and 97th percentiles. RCH's parent guide(opens in a new window) notes that most children grow between two of the curves rather than exactly on one.

Your baby's record book carries these charts. The NSW Blue Book(opens in a new window) has a head circumference-for-age chart for girls and one for boys, and the check sheets from 1 to 4 weeks to 12 months each have a line for head circumference in centimetres and as a percentile.

Other states use their own books and colours, listed in our guide to health checks in each state.

For a baby born before 37 weeks, WA nurses start on a preterm chart and move to the WHO head chart from 40 weeks corrected age. The head is then plotted at the corrected age until two, the same approach set out in the corrected age section.

Reading a head percentile

• Steady, 3rd to 97th: The head is growing along its own curve. It is measured again at the next routine check.
• Between two lines: Normal, as most babies track between the printed curves. Nothing extra happens.
• One odd reading: Often the tape position, thick hair or a wriggle. The nurse measures again to check.
• Above the 97th: A big head for age, which often runs in the family. The usual next step is a referral to a doctor for a closer look.
• Below the 3rd: A small head for age. The usual next step is a referral to a doctor for a closer look.
• Moving across lines: Head growth speeding up or slowing down over several checks. The next step is further assessment or a referral.
• Out of step with body: The head is much bigger or smaller than weight and length suggest. The usual next step is a referral to a doctor.

The referral triggers in this list come from the WA head circumference procedure, which lists a head below the 3rd or above the 97th percentile, a head out of proportion to length or weight, and serial measurements crossing two or more centiles downwards.

A referral is a request for a doctor's opinion, not a diagnosis. The same procedure notes that a change is usually confirmed with measurements over several visits.

What a GP looks for

A GP or paediatrician reads the head line alongside everything else about your baby. The Perth Children's Hospital guideline for doctors(opens in a new window) and the WA nursing guidance point to four things:

• The trend over several checks. A head percentile that keeps climbing or keeps falling across the lines matters more than any single dot.
• A mismatch with weight and length. A head much bigger or smaller than the rest of the body suggests gets a closer look.
• The parents' heads. Big heads run in families, and the WA guidance names family macrocephaly as the most common cause of a large head. Before referring, Perth Children's Hospital asks GPs to measure both parents' heads and plot them against the baby's.
• How your baby is otherwise. The RCH key messages on growth(opens in a new window) say a growth chart is not a diagnostic tool on its own, and ask whether a child is otherwise happy, healthy and meeting developmental milestones.

A flat spot is a separate question from head size. The WA procedure describes positional flattening, from lying in the same position, as a head shape change that does not affect brain growth or development. Mention it at a check, but it does not move the percentile.

When a head change is urgent

Do not wait for the next check if your baby is unwell and the soft spot on top of the head (the fontanelle) is bulging. healthdirect(opens in a new window) lists a bulging fontanelle among the signs of meningitis in babies, with fever or a low temperature, drowsiness or being hard to wake, poor feeding, floppiness or stiffness, vomiting and seizures.

• Go to the nearest emergency department or call triple zero (000): If your baby is unwell and the fontanelle is bulging.
• Book in with your GP or child and family health nurse: If your baby is well but a head measurement worries you. You can also call Pregnancy, Birth and Baby(opens in a new window) on 1800 882 436 to speak with a maternal child health nurse.

Perth Children's Hospital also asks doctors to discuss a baby with the on-call paediatric team for a bulging fontanelle, abnormal eye movements, or excessive vomiting, sleepiness, irritability or seizures.

Seeing the trend without the maths

The catch with trends is that they only exist if the measurements are written down somewhere — and the dots in the paper book are weeks apart and easy to lose track of between visits.

This is the boring problem BabyLog solves. Log a weight, length or head circumference in a couple of taps — after a nurse visit, or your own bathroom-scale check between appointments — and the growth view charts the trend for you, so you're looking at a line, not a number you half-remember from the green book. Because BabyLog is local-first, the chart loads instantly and works offline, and anyone you share care with — partner, grandparents — sees the same picture. The same data feeds BabyLog Insights, which looks at patterns across feeds, sleep and growth rather than any single day.

Track growth free — the growth chart is part of the free tier, no card required.

Growth is also only one thread of the first-year story; if you're wondering what your baby should be doing at each age, our guide to baby milestones by age in Australia covers the developmental side the same way — ranges, not races.

One last time, because it's the whole article in a sentence: the percentile tells you where your baby sits; the trend tells you how they're going. Only the second one is worth worrying about.

Common questions