When your pediatrician says your baby is at the 40th percentile for weight, they mean: out of 100 healthy babies of the same age and sex, about 40 weigh less. It is a position, not a score — there is no "pass" percentile.
How the WHO chart works
GrowChart and your pediatrician use the WHO Child Growth Standards (2006), built from healthy breastfed infants. Every age has a median (the 50th percentile) and a spread. A baby's weight is converted to a percentile or z-score using the WHO LMS method.
The 3rd to 97th band is "normal"
In practice, weights between the 3rd and 97th percentile (z-score roughly -2 to +2) are considered within the normal range for a healthy child. Below the 3rd percentile or above the 97th is worth discussing with your pediatrician — not because it is automatically a problem, but because it deserves a look.
What matters more than one dot
A single reading says less than the curve. Pediatricians look at whether your baby tracks along a percentile band over time. A baby who stays near the 20th percentile through the first year is different from one who drops from the 50th to the 5th in three months — the steady curve is reassuring, the drop is a reason to talk.
Track it yourself
GrowChart plots your baby's weight on the official WHO charts, computes the exact percentile and z-score, and compares it across visits — the same math your pediatrician uses. Create a free account and plot your first measurement.
Try the free calculator
No sign-up needed — check a percentile or z-score in seconds, then save & track when you create a free account.
Open the calculator → Create a free account