The red book you are given when you have a baby contains growth charts, and they are one of the most misread documents in parenting. So it is worth going through what they actually show.
What is in the book
There are different growth charts for boys and girls. Your baby is weighed at birth, and then again within the first two weeks of life to check they are regaining their birth weight. Some babies never lose it and simply continue to gain.
After that, the health visitor checks weights infrequently, unless you are attending hospital for regular appointments or there are concerns about growth.
Premature infants have their own growth charts, and growth is corrected depending on how premature the infant was, until the age of two.
Reading a centile
Each of the lines on the chart is called a centile. On a standard chart you will see head circumference at the top, length in the middle, and weight at the bottom.
If your baby's weight is plotted on the 25th centile, that means if you had 100 children, a quarter would weigh less and three quarters would weigh more.
If their length is on the 50th centile, that is the middle of the range.
Neither of those is better or worse than the other. A centile is a position, not a score.
Weight and height do not have to match
It is absolutely fine if your child is one centile above or below on their weight compared to their height.
This is one of the most common causes of unnecessary worry. Children are not required to sit on the same line for everything.
Growth fluctuates
All children grow at very different rates, and growth can fluctuate up and down over the months. A single plot slightly off where you expected is not usually meaningful.
When it does need looking at
A drop of more than two centiles would probably need to be looked at, and you should seek advice from a professional.
Growing below the 0.4th centile, which is the very bottom line on the chart, is also a reason to get advice.
How measurements should be taken
Technique matters, which is why home measurements can be misleading.
In the under twos, babies need to be measured lying down, with no clothes or nappies, for both weight and length. A length board is normally used, because length is very difficult to get accurately without one. This is usually done in a professional setting for that reason.
Why BMI is not used
As adults you are probably familiar with body mass index. We do not look at this in the under twos.
BMI cutoffs are also very different for children, so you cannot simply apply the adult cutoffs. There are specific charts for looking at BMI in children, but we mainly look at it in the context of obesity.
The thing to take away
The chart is a tracking tool, not a target. What matters is the pattern over time rather than any single point, and the specific things worth acting on are a drop of more than two centiles or growth below the 0.4th centile.
Everything else is usually your baby being your baby.
