When your paediatrician mentions that your child is "at the 25th percentile," what does that actually mean? And what is the z-score they sometimes write on the chart? These numbers matter more than most parents realise — and understanding them can help you have better conversations with your doctor.
What is a Percentile?
A percentile tells you how your child compares to other children of the same age and sex. If your child is at the 50th percentile for weight, it means they weigh more than 50% of children their age — right in the middle. At the 25th percentile, they weigh more than 25% — lighter than average, but still considered normal.
What is a Z-Score?
A z-score (also called Standard Deviation Score or SDS) is the more precise clinical measurement behind percentiles. It tells you how many standard deviations a child's measurement is from the WHO median for their age.
| Z-Score | Percentile (approx) | Interpretation |
|---|---|---|
| +2 SD | 97th | Above average (tall/heavy) |
| +1 SD | 84th | Slightly above average |
| 0 SD | 50th | Median (average) |
| -1 SD | 16th | Slightly below average |
| -2 SD | 3rd | Low — paediatrician review recommended |
| -3 SD | <1st | Severely low — medical investigation needed |
How WHO Z-Scores Are Calculated
GrowChart uses the LMS (Box-Cox power) method defined by WHO. Each measurement is compared to WHO reference values for L (skewness), M (median), and S (coefficient of variation) for that age and sex. The formula is:
Z = [(measurement / M)^L − 1] / (L × S)
This accounts for the fact that growth data is not perfectly symmetrical — babies grow quickly in some periods and slowly in others.
What Should I Watch For?
A single measurement tells you little. The trend over time is what matters:
- A z-score between -2 and +2 is considered normal range
- A drop of more than 1 SD in 3 months warrants discussion with your paediatrician
- A z-score below -2 is flagged by WHO as "underweight" or "stunted" depending on the indicator
How GrowChart Helps
GrowChart automatically calculates z-scores and percentiles for every measurement you enter, using the official WHO 2006 and 2007 reference tables. It plots your child's trajectory on a chart and highlights any readings outside the normal range — so you can bring accurate data to your next paediatrician visit.