The Child Growth Foundation, a charity set up to publicise the importance of monitoring growth, has published charts, recording the Body Mass Index (BMI) for boys and girls in the UK, showing what is average, what is obese and what is significantly underweight.
While many adults know their own BMI, because they want to discover whether they are thinner or fatter than average, few people know their children's index. And if they do know it, it has not been much use, because there has been no way of comparing it with that of other children of the same age.
Parents have been able only to consult height and weight charts, to see how their children measure up. These charts can be difficult to interpret if your child is in different positions on the two different graphs. If, for example, your son is much taller than most boys (among the top 90 per cent for his age), but about average for his weight, does that indicate that he is not eating enough? Not eating the right things? Or failing to absorb the food properly? Should you take him to see a doctor?
Now such a parent can measure their child's BMI (see box) and see whether he or she is within the range of normality; between, say, the bottom 2 per cent of the population and the top 2 per cent. Tam Fry, honorary chairman of the Child Growth Foundation, thinks the charts will be particularly useful in identifying obesity. "Obesity among children in this country is growing by 6 per cent, every five years," he says.
"It is often possible for doctors to spot the warning signs early on. The usual pattern of development in children is for them to put on a lot of weight in the first nine months, to lose it as they come out of infancy, and then to pick it up again in early or mid-childhood (about the age of six). That is known as adiposity rebound. Research shows that those children who start to pick it up again early (at the age of four or five), are likely to end up obese.
"Our charts enable doctors to spot this early, and warn parents that they should encourage their children to eat more healthily," Mr Fry says.
He thinks that parents are going to have to monitor their children more closely in future, because many health authorities are cutting back on health visitors and school nurses, or insisting that health visitors concentrate on those families most in need of help.
"Cambridge and Huntingdon Health Commission, for example, has said that they are not going to monitor children's growth after the age of two, which I think is criminal," he says.
"We are trying to find ways of making these charts more parent-friendly, and making them available through the Internet, or through a home computer package. We would like to design a package which could do some of the sums for parents, so that they could work out their children's BMI without too much difficulty."
The new charts were created using data from several UK surveys, covering almost 15,000 children. Dr Tim Cole, senior statistician at the Institute of Child Health, London, who led the team that devised them, thinks they are useful for international comparisons and for tracking trends over time. He also thinks they could be useful diagnostically, alerting professionals if a child's BMI changes suddenly relative to his peers.
"If a child's BMI starts crossing from one centile line to another, either upwards or downwards, it would be useful for doctors to be aware of it," he says.
For example, if a child's BMI had been going along the 50th centile for several years - which means about half the population is fatter than him and about half thinner - and then fell to the 2nd centile - which means 98 per cent of the population is fatter - it could mean that the child was developing a problem.
A parent who knows what it is to have children of very different shapes is Jill Satin, of Tufnell Park, north London. Her eight-year-old son, Max, has a BMI of 17.7, which is on the 75th centile for his age - which means that three-quarters of the children of his age are thinner - while her four-year-old son, Nathan, has a BMI of only 13.2, which is on the 2nd centile for his age - meaning that less than 2 per cent of children his age are thinner.
"I think these charts could be useful, as long as they are not used by professionals to make parents feel guilty," says Jill Satin. "I had a health visitor once who asked me whether I knew how to cook when she discovered how light Nathan was. I was insulted because I know a great deal about food and nutrition, and take a great deal of trouble about my children's diets.
"I know that Nathan is very light. Friends pick him up and say that he is lighter than their two-year-olds, but that has always been the case. Both my boys have asthma and see a paediatrician every three months, so they are well monitored."
Not all doctors think that these charts will be useful as a diagnostic tool, however. Dr David Jewell, a GP and consultant senior lecturer in primary health care at Bristol University, says: "You can usually tell if a child is overweight or underweight just by looking at him or her.
"The difficulty is not diagnosing the problem, but knowing what to do about it. If a child is overweight, it is often because the parents are over-feeding him or her, and it is usually difficult to persuade them to do otherwise. Anyone who has tried to lose weight knows that it involves hunger and denial. Parents usually do not want to see their children go through that. They find it difficult to refuse their children food.
"With children who are thin and eat little, I can usually be reassuring. Some children are naturally thin and they are lucky, because they are never likely to be overweight.
"It is common for children not to eat much, and I encourage parents not to turn it into an issue. Children seem able to live on relatively little food, possibly for evolutionary reasons. When we were hunter-gatherers, children probably had to survive on scraps from about the age of two, when breast-feeding stopped, to about 14 or 15, when they could find their own food."
Dr Jewell admits, however, that such charts could be useful for epidemiological reasons (for plotting population changes), and for parents confused by the fact that their children are in different positions on the height and weight charts.
"You could work out a child's BMI, and then see whether it was within the normal range. If it was, that could be reassuring to parents," he says.
How To Work Out BMI
To discover your Body Mass Index, measure your weight and height in kilos and metres then divide your weight by your height squared.
For example, if you weigh 82kg and you are 1.82m tall, the calculation is 82 divided by 1.82 squared (3.31) which comes to 24.77.
For adults, the ideal BMI is between 18.5 and 24.9, between 25 and 29.9 means pre-obesity, and over 30 is obese.
For children, see chart left, between the ages of two and 17, the ideal figures range between about 13 and 27, according to age.