Interpretation of centiles

It is dangerous to take an arbitrary figure such as the 10th or even 3rd centile and to declare that anything below that is abnormal. Not only does this automatically label 3% or even 10% of the population as "abnormal" but some cases of abnormality will be missed. What matters is not so much an isolated value as changes with time.

Suppose that an ultrasound scan at 32 weeks' gestation shows that a fetus 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. The obstetrician will not wait for either parameter to fall to the "abnormal" level of the 10th or 3rd centile as there is already compelling evidence of intrauterine growth retardation and the fetus is at risk. Delivery should be expedited.

The baby is delivered and weight is on the 10th centile. Over the next few weeks the baby gains weight but how is he or she doing? If their weight stays on the 10th centile they are maintaining the status quo. Their weight may rise but slower than expected so that it starts to fall towards the 3rd centile. This is cause for concern as there is failure to thrive. Babies with intrauterine growth retardation are usually hungry, gain weight fast and their weight rises through the centiles.

Recognising childhood obesity

A child may be brought to the consultation with a complaint of being overweight, but more often this is noticed during the consultation and it may be difficult to convince the parents and child that a problem exists.

BMI charts should be used with great care and not without interpretation until at least 16 and possibly 18 years of age:

• A BMI of 20 for young teenagers is too heavy and before puberty BMI should be substantially less.
• The UK 90 charts do have a component for the measurement of BMI in children and this has been validated.^[3]
• The Royal College of Paediatricians and Child Health has adopted new charts that combine UK90 and World Health Organisation (WHO) data.^[4]

There are often excuses and denial such as "He is big for his age", "It's just puppy fat", "He'll grow out of it" and "We are all big in our family" (we all overeat together).

Research suggests that overweight children are unhappy children^[5] and that they become overweight, unhappy and unhealthy teenagers and adults.^[6] Both undernutrition and obesity are more common with social deprivation.^[7]

Management

• Record height and weight but ignore the calculation of BMI.
• Use centile charts to find those two parameters for that child's age.
• There are different charts for boys and girls.
• Fat children are usually tall for their age and so they will be high in the height centiles, but they will be even higher in the weight centiles.
• Thus a child whose height is on the 75th centile and weight over the 97th centile is obese.
• If a child is overweight but not tall for age, this is a cause for concern.

The approach to weight loss for children is also different from adults.^[8][9] It may not be necessary or desirable that the child actually lose weight. Instead, the gain in weight should be so slow that the child's weight falls through the centiles and approaches the same centile as height.

Inadequate growth

A child may be brought with a complaint that he or she is too small. Note any medical history that may suggest a cause:^[10]

• Ask about diet and eating habits and weigh and measure the child.
• Again, look at the centile charts; if the child has height on the 25th centile and weight between the 3rd and 10th centile, this does suggest poor nutrition.
• Ask the height of the parents; tall parents can expect to have tall children and short parents have short children.

It may be that there is insufficient cause for concern to justify immediate referral to a paediatrician and a period of observation is required:

• Make serial recordings of height and weight and plot them on centile charts. If the child continues along the same centile, this is probably just constitutional smallness.
• He may catch up and show some rise through the centiles.
• If he falls further through the centiles, even if he is gaining height and weight, this is a cause for concern and a reason to refer.

Too fat or too thin?

A child may be brought with complaints that he or she is too fat or too thin:

• Ask the child how he or she feels about body size.
• Do they agree with the parent that they are too thin?
• Again record height and weight and check centile charts. Height and weight do not have to be on exactly the same centiles but so long as they are in the same order of magnitude, this is reassuring.
• Parents may be trying to overfeed the child but also eating disorders can start young.
• Ask the child about any teasing at school about weight.

Do not be afraid to express your opinion about whether the child looks too fat, too thin or absolutely fine.

Creating centile charts

Centile charts are supposed to demonstrate the statistical scatter of the normal population. Therefore, a great many individuals must have the relevant parameter measured so that the sample size is large enough to be statistically rigorous. They are supposed to represent the normal population and so those with obvious pathology should be excluded from the data to produce the charts. This may include children with a syndrome such as Down's syndrome or achondroplasia. It may include those with diseases that lead to malnutrition of recurrent infection including coeliac disease or cystic fibrosis or cyanotic congenital heart disease.

There is also a question of whether or not to include those who are simply too fat. Unless they have a disease, they should probably be included although if their numbers are great they will skew the distribution.

Problems using centile charts

• If centile charts are based on the normal population and they are being used mostly for children with disease, who are not part of the normal population, are they valid? The answer if probably that they are the best available tool but their limitations must be remembered. There are specific reference charts for Down's syndrome and Turner's syndrome. Charts for very low birthweight infants are proving problematic.^[11]
• Are they applicable if based on one race but used in another? Variations within races are greater than variations between races and so they are probably applicable but with the usual caveats, including asking about the height of parents.
• Until 2006 growth charts were based on children with mixed feeding patterns, predominantly bottle fed, but evidence from various studies suggested that exclusively breastfed infants gain weight differently:^[12]
  • The worry has been that misinterpretation of growth charts could lead to breastfed babies being given unnecessary supplements of formula.
  • This has led the WHO to develop new charts, which have been adopted for all children from 0 to 4 years (see below).
  • After these charts are adopted fewer infants will be defined as underweight, whereas the proportion who are overweight will increase.^[13]
• Many of the height and weight centile charts still in use today date back to 1956:
  • Children are bigger now than they were then. They grow into bigger adults and so the charts, especially for older children, may not be an accurate reflection of modern childhood. Although overweight children will be big for their age, modern children of correct proportion grow into significantly taller adults than in the 1950s.
  • Obese children were also rather uncommon in the 1950s. They are much more common today and if 20% of children are overweight or obese, this will skew the upper reaches of any charts based on modern children.^[14] A child on the 90th centile for height and weight on a modern chart may not be as ideal as the correlation for height and weight suggests.
  • A study from Liverpool suggested that between 1989 and 1998, the number of overweight children increased from 14.7 to 23.6% and the proportion who were obese rose from 5.4 to 9.2%.^[15]
• It is not only height and weight charts where old references are a problem. If the Gardner-Pearson chart is used to assess the head circumference of boys at 6 months, the average modern boy is on the 75th centile.^[16]
• Puberty is a time of rapid growth but it is variable in onset. Hence it is to be expected that children will cross centile lines around this time. Those with an early puberty will be climbing through them whilst those awaiting puberty will fall through them as their peers reach their growth spurt earlier.
• The WHO is eager that updated charts should be used and the Royal College of Paediatricians and Child Health has now adopted them:^[4]
  • The College has previously recommended that the Tanner and Whitehouse references should be discarded in favour of the Buckler or UK90. Their initial reluctance to change may relate to relatively recent acceptance of the latest UK90 update and validation.^[17]
  • If charts are changed too often it is difficult to map secular changes of height, weight and obesity with time.