Understanding growth charts and BMI Part 2

Posted by Kath Megaw on 30/09/2015

What the 50th percentile (average line) actually means

The 50th percentile is not the “perfect average” that your child needs to reach! The 50th percentile (or the 0 curve) is merely that weight where half the normal population weighs more, and half the normal population weighs less. Similarly, the 15th percentile is the weight at which 85% of the population weighs more, and 15% of the population weighs less.


When to worry and when not to!

Do not worry if:

  • Your child is born at a weight or length outside of the “green” range, and in the dreaded “orange or red zone” (below the -1 curve, or above the +1 curve). As long as your child is healthy in all other ways, it probably just means your baby is genetically small or large.
  • Your small child stays small or large throughout childhood. As long as their growth curve is similar to the curve of the chart, they are growing well. Check their weight for length chart for better clarification of what their weight really means.
  • Your child’s curve fluctuates over time but has a general trend that is similar to the curve of the chart.
  • Your premie child picks up weight quickly and then settles into a new growth curve – this is common in premies and is simply catch-up growth.
  • Your child shoots up in length and weight. Height is largely genetically determined, as long as their weight for length is still normal this is fine.


Seek out a health professional if:

  • Your child is steadily losing or gaining weight to the point of crossing a growth curve
  • Your child is steadily picking up weight, just not as fast as the growth curve, for two weigh-ins or more
  • Your child is not picking up weight and has a “flat” growth curve for two weigh-ins or more
  • Your child was born below the -2 or -3 curve and is losing weight
  • Your child was born above the +2 or +3 curve and is gaining weight faster than the curve


How accurate are BMI measurements in children

As adults, we measure ourselves against a healthy BMI of 18-25 (see equation above). A value less or more than this range indicates that we are under- or overweight. With children, it is not that simple. Their little bodies are going through so many changes from 5-19 years that we cannot compare them to an adult BMI. For this reason, the BMI for age charts were developed, to give us a more reasonable expectation of their weight and height. For example, a BMI of 16 for a 10 year old is perfectly healthy. Please note: when it comes to your child’s BMI chart, this is one case where we do actually want your child within the -1 and +1 growth chart – if your child consistently falls outside of these curves, please seek out advice from a healthcare professional.

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