What is a centile? How to understand the centile graph

If you’ve taken your babs for a check-up, you’ll know that the docs measure growth according to a graph divided into centiles. I remember being told that my daughter was on the 50th centile for her height and I nodded, smiled and had no freakin’ clue what that meant.

Um, what is a centile exactly? Two children later, I thought it was about time I learnt.


… In an effort to monitor the development of your baby (or foetus) doctors will record measurements and plot them on a graph that shows a number of lines representing centiles; height, weight or head circumference are shown on the y axis and age or gestation on the x-axis.

‘Centiles’ are units of measurement that show how your child is developing in relation to children of the same age. If your child is on the 60th centile for her height, it means that when 100 girls of her age are arranged in ascending order of height, your daughter will stand in 60th position  – she’s shorter than 61 but taller than 49. If your child is closer to the bottom end of the ‘centile’ graph, she is probably quite petite and if she’s on the top end of the ‘centile’ graph she’s most likely fairly big.

Taking into consideration the above explanation, it seems logical to conclude that any child not falling into a position on the centile graph is abnormal? I mean, that’s how I interpret it. The truth is that doctors are never quick to label children ‘abnormal’ but what you can be sure of is that if they feel that your child’s development is not as it should be, they will investigate the matter.

As far as babies in utero are concerned, the big deal is that the measurements are proportionate. My second baby’s head did not fit on the centile graph (and her body did). The docs kept an eye on her growth and were not too concerned because her head and body grew proportionately as she got older, and she was perfectly ‘normal’ at birth. The time to worry is when growth (or lack thereof) does not happen proportionately and consistently. Patient.co.uk explains it as follows:

“Suppose that an ultrasound scan at 32 weeks’ gestation shows that a foetus has a head and abdominal circumference both on the 50th centile. The scan is repeated at 36 weeks and, whilst both circumferences have grown, the head circumference has fallen to the 25th centile and the abdominal circumference is a little above the 10th centile.”

In this case, the docs will then investigate the strange growth pattern.

If you want to understand the centile graph, and even my explanation is confusing, don’t feel silly asking your GP! – Docs will assume that we understand what’s potting unless we say otherwise.

Sources: Patient.co.uk – “Centile Charts and Assessing Growth” and “Prima Baby & Pregnancy” March 2013 – “Decoding the Red Book” p36-63