The charts in the child health booklet
In Germany's yellow child health booklet, parents find charts with several curved lines. At every check-up, the paediatrician enters height, weight and often BMI there. These lines are percentile curves. They show how the respective values are distributed among healthy children of the same age and sex. If you can read the curves, you will better understand what is discussed at the check-up.
What the individual lines mean
Each line stands for a percentile. The usual ones are the 3rd, 10th, 25th, 50th, 75th, 90th and 97th percentiles. The middle line, the 50th percentile, is the median: half of the children lie below it, the other half above. The outer lines mark the areas where only a few children lie. Three in 100 children are below the 3rd percentile, and three in 100 above the 97th.
For BMI the lines have a special meaning. According to the guideline of the German Working Group on Obesity in Childhood and Adolescence (AGA), the 90th percentile marks the threshold for overweight and the 97th the threshold for obesity. At the lower end, the 10th percentile is the threshold for underweight and the 3rd for severe underweight.
How to read a point
The horizontal axis shows age, the vertical axis the measured value. First find your child's age, then go straight up to the measured value. The point usually lies between two lines. If, for example, it lies between the 50th and 75th percentile, your child has a slightly higher value than average but is clearly within the healthy range.
Reading exact figures off the chart can be fiddly. That is why the child BMI calculator calculates the percentile directly and plots your child's point on the curves.
The trend counts more than a single point
The most important rule when reading the curves: a single point is a snapshot. The chart becomes meaningful only once several measurements have been entered. Most children move roughly parallel to a particular line for years. A child who always grows around the 25th percentile is simply a little more slender than average — and that is perfectly fine.
Parents should pay attention if the trend clearly changes direction. If the curve crosses two or more percentile lines up or down within a short time, the cause should be clarified. It may be entirely harmless, such as an early growth spurt. But it can also be a sign that something should be looked at more closely.
Looking at height, weight and BMI together
The booklet usually has separate curves for height and weight. Both values are linked: a tall child usually also weighs more. BMI combines the two and makes children of different heights comparable. A child on the 90th percentile for both height and weight may well end up in the middle range for BMI.
Typical patterns in the curves
In the first year of life BMI rises steeply, then falls until around the age of four or five before increasing again. This dip is normal and built into the curves. During puberty the curves for girls and boys differ because development starts at different times. Between roughly 11 and 16, girls have a slightly higher median BMI than boys; after that the relationship reverses.
If questions remain
Percentile curves are a tool for conversation, not a diagnosis. If you feel unsure when looking at the booklet, ask at the next check-up. Paediatricians are happy to explain how they assess your child's development. And if your child is already an adult, the adult BMI calculator will help.