"Your baby is in the 20th percentile" is a phrase a lot of parents hear at a checkup and leave without quite understanding if that's good, bad, or neither. Spoiler: it's almost always the third one. Here's the full explanation, no fluff.
What WHO growth curves are
The World Health Organization's (WHO) growth curves are the reference standard pediatricians worldwide use to track how a baby grows over time. They were built from data on thousands of healthy babies, raised in favorable conditions (good nutrition, no chronic illness, access to medical checkups), across six different countries. That's why they're a standard for how a baby should grow under optimal conditions, not an average of how every real baby actually grows.
How to read a percentile (the part nobody explains well)
A percentile compares your baby to other babies of the same age and sex. If your baby is in the 40th percentile for weight, that means 40 out of 100 babies that age weigh less than yours, and 60 weigh more. The 50th percentile is the median — not "the ideal average" or a goal to reach, simply the midpoint.
The curves usually show a range from P3 to P97: being anywhere within that range is, statistically, expected. There's no "good" or "bad" percentile — a healthy, well-fed baby can perfectly well sit in the 10th percentile their whole childhood, if that's their own individual curve.
The 4 curves that matter
The WHO (and most tracking apps, Alfonso included) work with four different curves, each measuring something different:
- Weight-for-age — the best known, but the one that says the least on its own.
- Height-for-age — reflects longer-term growth than weight.
- Head-circumference-for-age — tracks head growth, relevant for early neurological development.
- Weight-for-height — compares weight against height, not age; helps see whether weight is proportional to your baby's size, regardless of how many months old they are.
Weight/age curve with WHO percentiles — a real screen from the Alfonso app.
What's actually worth watching: the trend, not the number
A single percentile, on its own, says little. What pediatricians watch more closely is the trend over time: a baby who's been growing steadily along the 30th percentile for months is following their own curve normally. A baby who drops sharply from the 60th percentile to the 15th in a short time is what's worth checking with the pediatrician — not the number itself, but the sharp shift.
That's why a single measurement isn't worth much on its own, and the real value is in logging every checkup so you can see the full chart, not one isolated point.
Start logging today
Create your baby's profile in 30 seconds, no account or password needed, and log the next event as it happens — to see the real pattern, not just remember it. Syncs automatically with your partner.
Frequently asked questions
What if my baby is in the 10th percentile?
Nothing, on its own. The 10th percentile means 10 out of 100 babies that age weigh less and 90 weigh more — it's a point within the expected range (P3 to P97), not a problem. What a pediatrician watches more closely is whether that percentile stays stable over time or shifts sharply.
Are the curves the same for girls and boys?
No. The WHO publishes separate curves by sex because growth pace differs slightly between girls and boys from birth. Using the correct curve for your baby's sex matters for the percentile to actually mean something.
How often should I measure my baby?
That schedule is set by your pediatrician based on routine checkups (usually more frequent in the first year). What matters for the curves isn't measuring often, it's logging every real measurement so you can see the trend over time.
Keep reading
ℹ️ This article is informational and doesn't replace a pediatrician's judgment. If you have any concern about your baby's growth, always check with a healthcare professional.