A growth percentile compares your baby's weight, height, or head circumference to a reference population of babies the same age and sex - it says nothing about whether your baby is "big" or "small" in any absolute sense. A baby at the 40th percentile for weight is, by definition, perfectly typical: heavier than 40% of babies their age and sex, lighter than the other 60%.
Note: This article explains how percentiles work in general. It isn't medical advice - only your pediatrician can interpret your baby's specific growth chart.
What a Percentile Actually Means
Percentile charts are built from measurements of a large reference population of healthy children at each age. If your baby's weight sits at the 65th percentile, it means their weight is higher than 65 out of 100 babies of the same age and sex in that reference group - nothing more. There's no "good" or "bad" percentile; 50th isn't a target, and neither is any other number.
Why the Trend Matters More Than the Number
Pediatricians generally care much more about whether a baby is tracking consistently along their own curve over time than about which percentile line that curve happens to follow. A baby who has measured around the 30th percentile for weight at every checkup is typically considered to be growing right on schedule - for them. A sustained or notable change in the curve can prompt a closer look, because the pattern across accurate measurements is more informative than one isolated percentile value.
WHO Growth Standards vs. Other Charts
Different growth chart standards exist. The World Health Organization's Child Growth Standards, built from data on healthily fed children up to age 5, are the standard commonly recommended for children under 2 years old. Other charts (like growth references built from broader population data) are more commonly used for older children. Which chart is used can shift a baby's percentile number somewhat, which is another reason the exact number matters less than the trend on a single, consistent chart over time.
When a Percentile Change Is Worth Mentioning to Your Doctor
- A sustained or notable shift in the curve across accurate measurements
- Weight percentile dropping noticeably while height keeps tracking normally, or vice versa
- A plateau in weight or height gain over several consecutive checkups
- Any change alongside feeding difficulties, unusual sleepiness, or other symptoms
None of these automatically mean something is wrong - kids grow unevenly sometimes, especially around illnesses or feeding changes. They're simply worth raising at the next visit rather than waiting.
Frequently Asked Questions
Is a low percentile bad for a baby?
Not on its own. A baby who consistently tracks around the same percentile at every checkup is generally considered to be growing right on schedule for them - there's no "good" or "bad" percentile.
What growth chart does Milli use?
Milli charts growth against the World Health Organization's Child Growth Standards for children 0–24 months, for both boys and girls.
When should I be concerned about my baby's growth percentile?
A sustained or notable change in the curve, a plateau across several visits, or a change alongside feeding difficulties or other symptoms is worth raising with your pediatrician.
Sources
This guide was fact-checked against the following guidance on July 14, 2026:
Related Reading
How Milli Tracks This For You
Growth charts, built in
Milli logs weight, height, and head circumference and charts them against WHO Child Growth Standards percentiles for children 0–24 months, for both boys and girls - so you can see the trend over time instead of trying to remember last checkup's number.