When the pediatrician tells you “your baby is in the 35th percentile,” you feel like you should know what that means — but you’re not quite sure. Here’s the simple explanation.
What is a percentile?
A percentile is a relative position. If your baby is at the 35th percentile for weight, it means they weigh more than 35% of babies their age and sex, and less than the remaining 65%.
The important part: no percentile is “better” or “worse” on its own. A baby at p15 who grows consistently is just as healthy as one at p75. What matters is the trajectory, not the number.
Why the WHO, and not another organization?
The WHO (World Health Organization) published growth charts in 2006 based on babies from around the world raised in optimal conditions. They’re the international reference because they reflect a baby’s growth potential, not just the statistics of one specific population.
The percentile bands
The WHO charts show:
- p3: 3% of babies weigh less than this value
- p15: 15% weigh less
- p50: the median — half of babies weigh less, half weigh more
- p85: 85% weigh less
- p97: 97% weigh less
Being between p15 and p85 is the “typical” range, but being outside it isn’t necessarily a problem.
When should you actually worry?
The pediatrician will pay closer attention if:
- The baby suddenly drops below p3 or rises above p97
- The baby crosses two percentile lines downward in a short time
- Growth stalls for several weeks
A single point on the chart tells you nothing. You need the full trajectory.
Weight isn’t everything
The growth chart includes:
- Weight: the easiest to measure
- Length/height: important for the weight/height ratio
- Head circumference: reflects brain development
A tall, lean baby might be at p80 for height and p30 for weight, and be perfectly fine. That’s why the pediatrician looks at all three together.
How to use the app to track growth
Every time you log a weight, the app plots it automatically on the WHO curve. You can see:
- Where your baby sits relative to the median (p50)
- The trend across recent weigh-ins
- The difference in kg compared to the median for their age
Always bring the health record to appointments and log every weigh-in. With a complete history, the pediatrician can read the curve far more accurately.