How baby growth percentiles work
A growth chart shows how healthy babies of the same age and sex are spread out by weight, length and head size. Your baby's percentile is where they fall in that spread: at the 70th percentile for length, about 70 in 100 babies the same age are shorter and about 30 are longer. The 50th percentile is the median, the middle of the group.
A percentile is a snapshot. Pediatricians care more about the line your baby follows from one checkup to the next than about any single number, which is why they plot every visit on the same chart.
Why the WHO charts until age 2
The CDC recommends that US clinicians use the 2006 WHO growth standards for children under 24 months and the CDC growth charts from 2 to 19 years. The WHO charts were built from healthy, breastfed babies in six countries (Brazil, Ghana, India, Norway, Oman and the US), so they describe how babies grow in optimal conditions rather than how one population happened to grow. Around age 2, measurements also switch from length lying down to height standing up, which is when the chart changes.
What the 2nd and 98th percentile lines mean
With the WHO charts, the CDC recommends screening with the 2.3rd and 97.7th percentiles, two standard deviations either side of the median, printed on the charts as the 2nd and 98th. These lines are statistical, not health limits: the CDC itself notes they aren't based on health outcomes. A baby outside them gets a closer look, not a diagnosis.
| Result | What it usually means |
|---|---|
| Between the 2nd and 98th percentile | Within the range of the WHO reference babies. What matters is that your baby keeps following a similar line. |
| Below the 2nd or above the 98th | Worth raising at a checkup. Your pediatrician will look at the trend, feeding, health and family heights. |
| Crossing percentile lines over several visits | The pattern pediatricians watch most closely, in either direction. |
How the calculation works
The WHO describes each chart with three numbers for every month of age: L (how skewed the spread is), M (the median) and S (how wide it is). For your baby's exact age, the calculator takes those numbers from the CDC's WHO data files, interpolating between months, turns the measurement into a z-score with the LMS formula, z = ((value ÷ M)L − 1) ÷ (L × S), and converts the z-score into a percentile. For very high or low weights it uses the WHO's adjustment beyond three standard deviations, so the result stays sensible at the extremes.
Babies born early are compared by age from birth unless you enter their due date instead; our adjusted age calculator shows that age and explains when it's used.