What a Percentile Actually Says
A percentile compares a baby with others of the same age and sex. The 40th percentile for weight means 40% of babies weigh less and 60% weigh more. It is a position in a distribution, not a score — and by definition, half of all healthy babies are below the 50th percentile.
The WHO growth standards used here were built from breastfed infants raised in optimal conditions across six countries, and they describe how children should grow rather than how a particular population happens to grow.
The Trend Matters, Not the Number
A baby steadily tracking the 15th percentile is growing well. A baby dropping from the 75th to the 25th across several months is what prompts investigation, even though the 25th percentile is entirely normal in isolation.
Some crossing of lines is expected in the first two years as babies settle onto their genetically determined trajectory — large babies of small parents drift down, and small babies of large parents drift up. Crossing two major percentile lines is the usual threshold for a closer look.
Weight Loss After Birth Is Normal
Newborns typically lose 5–10% of birth weight in the first days as they shed fluid, regaining it by two weeks. Loss beyond 10%, or failure to regain birth weight by two weeks, warrants a feeding assessment.
Typical Growth in the First Year
| Age | Weight change | Length change |
|---|---|---|
| 0–3 months | About 30 g a day | 3–4 cm a month |
| 3–6 months | About 20 g a day | 2 cm a month |
| 6–12 months | About 12 g a day | 1–1.5 cm a month |
| By 5 months | Roughly double birth weight | — |
| By 12 months | Roughly triple birth weight | About 50% longer than at birth |
WHO or CDC Charts
The CDC recommends the WHO standards from birth to 24 months and its own charts from 2 to 19 years. The difference matters: CDC charts are based on a largely formula-fed US sample and show faster weight gain, so a breastfed baby tracking normally on WHO charts can appear to be faltering on CDC ones.
Frequently Asked Questions
My baby is on the 10th percentile — should I worry?
Not on that basis alone. Roughly one baby in ten is on or below the 10th percentile, and consistent growth along that line with normal development is reassuring.
Does a high percentile mean my baby will be large as an adult?
Weakly. Infant size correlates poorly with adult height; parental height is a far better predictor after the first two years.
Should I use corrected age for a premature baby?
Yes — age from the due date rather than the birth date, until about two years. Without that correction a premature baby will appear to be growing far below expectation.