Understanding growth charts and BMI Part 1

Posted by Kath Megaw on 15/09/2015

Every parent is familiar with the importance (and potential stress!) of your little one’s height and weight. Physical growth is not only an indicator of nutritional status, but can also reflect underlying illnesses and infections. For this reason, the growth chart is one of the most useful and simple tools in monitoring your little one’s health.


Science behind growth charts

Growth charts are made by collecting measurements from thousands of normal, healthy children across different age groups. Where possible, as much variety is included as possible – that means children from different socioeconomic backgrounds, different cultures and races, different countries. These measurements are then plotted on a graph to come up with the lines that you see on a growth chart.


Understanding the growth chart

Growth charts have two main purposes: Firstly, to give us an indication of where your child stands in comparison to the rest of the population. As you can see by the way the growth charts are constructed, they are simply a reflection of how most healthy, normal children grow. Bear this in mind when reading your child’s growth chart: The growth chart do not serve to show the “perfect weight” or “perfect height”, but merely show you where you child stands in comparison to most other healthy children.


The second purpose is more important: No matter where your child is in comparison to other children, is he or she growing at a good rate? The curves of the growth chart give us an idea of how quickly we want your little one to pick up weight. Typically, we want your child to stay on the same curve, or between the same two curves, throughout his/her infancy and childhood.


Which growth chart for my baby

The most widely used growth charts (including the ones in the RTHB) are those produced by the World Health Organisation (WHO). There are others available, for example the Centre for Disease Control (CDC) charts. The WHO charts are preferable in South Africa as the data was collected from many countries across the world, including 3rd world countries such as Brazil and India, meaning the populations are more comparable to our own.


There are several important growth charts to monitor throughout childhood:

  • The weight for age chart shows us how much “normal” children weigh at each age. This is typically used until 5 years of age.
  • The length for age chart shows us how long or tall “normal” children are at each age.  This is typically used until 2 years of age. After this you can use the height for age chart until 5 years of age.
  • The weight for length/height chart is an invaluable and often underappreciated chart. This assesses your child’s weight in comparison to their height. This can give more detail on your child’s growth – for example, if your child is picking up weight at a normal pace, but has shot up in height, we may actually see that your child is now a bit thin for their height. Alternatively, if your child has a normal weight for age but is in fact quite short, this chart may show us that your child is a bit chubby for their height! This can be used until 5 years of age.
  • After 5 years, you will stop using these traditional growth charts and start using BMI for age instead. BMI (body mass index) takes into account your child’s height and weight to get an idea of their nutritional status. You can calculate BMI using this equation:


BMI = Weight (kg) ÷ Height (m)2

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