Not a diagnostic tool. Growth charts are a screening aid. Only your pediatrician, who can weigh a measurement against your baby’s history and clinical picture, can say whether growth is a concern.
Percentiles are the most misread number in the first year. Parents hear “25th percentile” and think their baby is behind. They are not. A percentile is a position in a distribution, not a score.
What a percentile actually means
If your baby is on the 25th percentile for weight, it means that out of 100 healthy babies of the same age and sex, about 25 would weigh less and about 75 would weigh more. That is the entire claim. Someone has to be on the 5th percentile and someone has to be on the 95th, and both can be perfectly healthy babies.
The 50th percentile is the median, not a target. Half of all healthy babies are below it by definition.
The trend matters far more than the number
A baby who tracks steadily along the 10th percentile for a year is growing well. A baby who was on the 75th at two months and has fallen to the 25th by six months is the one worth a conversation, even though 25th is a “better” number.
What clinicians look for is crossing centile lines, especially downwards, and especially crossing two or more of the major lines. Small wobbles are normal — measurement is imprecise, particularly length in a wriggling baby, where a centimetre of error is easy.
Which chart is being used?
Most of the world, and the NapNap app, uses the WHO Child Growth Standards for the first two years. These are prescriptive: they describe how babies grow under optimal conditions, based on a study of healthy, breastfed babies across six countries. The CDC charts, used for older children in the US, are descriptive — they describe how a reference population actually grew.
The practical consequence: breastfed babies typically gain faster than the CDC reference in the first few months and slower after about three months. On the WHO charts, that pattern is the norm rather than a deviation.
How the numbers are actually calculated
Growth charts use the LMS method. For each age and sex, the WHO publishes three parameters: L (a skew factor), M (the median) and S (the coefficient of variation). A measurement is converted to a z-score:
Z = ((measurement / M)L − 1) / (L × S)
The z-score is then mapped onto a percentile through the normal distribution. A z-score of 0 is the 50th percentile, +1 is roughly the 84th, and −1 is roughly the 16th. This is why percentile lines are not evenly spaced on the chart: the distribution is skewed, and the L parameter accounts for it.
One measurement is a dot. Three are a line. NapNap plots your baby’s weight, height and head circumference against the WHO percentile curves as you record them, so you see the trend and not just today’s number.
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What gets measured, and why
- Weight for age — the most responsive measure, and the first to move if feeding is not going well. Also the noisiest: a full nappy or a recent feed shifts it.
- Length or height for age — slower to change and much harder to measure accurately in a baby. Treat single readings with caution.
- Head circumference for age — tracks brain growth. Consistency matters here; a sharp change in either direction is worth mentioning.
- Weight for length — puts weight in the context of size, and is often more informative than weight alone.
Normal things that look alarming
- Weight loss in the first week. Losing up to about 7–10% of birth weight is expected, with birth weight usually regained by around two weeks.
- Regression to the mean over the first months. Birth size reflects the pregnancy as much as the baby’s own trajectory. Babies born large often drift down a little, and babies born small drift up, over the first six months.
- A plateau during illness, with catch-up growth afterwards.
- A slowdown around 3–4 months in breastfed babies, which the WHO charts already account for.
When to ask your pediatrician
- Crossing two or more major percentile lines downwards
- No weight gain over several weeks, or any weight loss after the first two weeks
- Head circumference changing percentile sharply in either direction
- Growth concerns alongside feeding difficulties, low energy, or fewer wet nappies
- Any point at which you are worried — that is reason enough
If the question is feeding rather than growth, the milk amounts by age guide covers typical volumes and the signs that matter more than volume.
Sources
- World Health Organization, WHO Child Growth Standards (2006), weight-for-age, length/height-for-age and head-circumference-for-age tables.
- Cole TJ, The LMS method for constructing normalized growth standards, European Journal of Clinical Nutrition (1990).
Common questions
Is a low percentile bad?
No. A percentile is a position among healthy babies, not a grade. A baby who tracks steadily along the 10th percentile is growing well. What matters is the trend over time rather than the number at any single visit.
What does it mean if my baby drops percentiles?
Small fluctuations are normal and often just measurement error. Crossing two or more major percentile lines downwards is the pattern that warrants a conversation with your pediatrician, particularly if it happens over a short period or alongside feeding difficulties.
Why do WHO and CDC charts give different percentiles?
WHO charts are prescriptive, built from healthy breastfed babies raised in optimal conditions, and describe how babies should grow. CDC charts are descriptive, showing how a reference population did grow. Breastfed babies typically appear to gain faster early and slower after about three months on CDC charts, which the WHO charts treat as normal.
How is a growth percentile calculated?
Charts use the LMS method. For each age and sex there are three published parameters, L, M and S, which convert a measurement into a z-score using the formula Z = ((measurement/M)^L − 1) / (L × S). The z-score is then mapped onto a percentile through the normal distribution.
How often should my baby be weighed?
Routine checks at your normal well-baby visits are enough for most babies. Weighing very frequently at home tends to add anxiety without adding information, because day-to-day variation from feeds and nappies is larger than the real trend.