In the US last week a distraught family told the media how their three-year-old girl was taken into care. "They dragged her out of the room kicking and screaming," said her mother. "She was terrified." The toddler in question, Anamarie Martinez-Regino, weighs eight and a half stone. She is three times as heavy as an average American three-year-old and, according to the doctors involved, her condition is "life-threatening".

Childhood obesity is a severe problem in the US. The latest data shows that more than one-fifth of US children are overweight, and about one in 10 obese. You might think this predictable in the land that invented doublechocolatechocolatechip ice-cream. But look around you: British kids, too, are getting fatter.

Last year, a study in the British Medical Journal found that nearly 16% of two-year-olds were overweight and 6% were obese, while more than 20% of four-year-olds were overweight and 7.6% obese.

This hasn't always been the case: between 1972 and 1994, English boys grew 10% fatter, and girls 5%; while in Scotland boys got 13% fatter and girls 16%.

Of course, there's nothing wrong with a chubby child. But a heavily overweight one is a different matter. Obesity in children can cause health problems: an increased risk of disease, problems moving around, a greater risk of cardiovascular difficulties and of abnormalities in glucose metabolism (diabetes, for instance).

This doesn't even touch on the psychological damage a fat child can suffer. "Every minute of every day friends, teachers, family, TV and magazines are telling them they're worthless," says Dr Paul Gateley, senior lecturer in exercise physiology and health at Leeds Metropolitan University.

So what should you do if your child is worryingly fat? Should you tackle the issue and risk undermining their self-esteem, or sit tight and hope it will sort itself out?

"Children grow at different rates," Gateley says. "They often lay down fat in preparation for growth spurts."

"The first thing you should do if you're worrying," says Dr Susan Jebb, head of nutrition and health at the Medical Research Council's nutrition centre in Cambridge, "is compare your child with others of the same age. If yours is significantly fatter (say one of the fattest three), then you may have cause for concern." Then you should see your GP. "They'll compare your child's size to 'normal' growth charts, and establish whether there is genuinely a problem."

But the chances are nobody will be able to tell you exactly why your child is fat. Parents will often put a child's (and their own) weight problem down to "unlucky genes". They'll claim that their little one eats normally, and yet still gains weight. Genes certainly could play a part in deciding fatness: numerous studies have clearly shown this. But the findings are far from specific. "So far more than 200 genes have been identified that have some association with obesity," Jebb says. "But we really know very little about the genes themselves."

What we do know is that it's rare for genes alone to cause a child to balloon. Only five children worldwide have been identified in whom there is a specific defect in the "ob gene". This single defect can lead to gross obesity from an early age (and could well be the cause of Anamarie's weight gain).

But with most fat children, the diagnosis is more straightforward. "If your energy intake exceeds your energy needs," Jebb says, "you'll gain weight." Not surprisingly, research has shown a marked similarity between the eating habits of parents and their children. Wolfing a Wispa while trying to get your kids to eat broccoli, is not the best strategy. "You have to create the right environment for healthy eating," says Gateley, "one in which your children eat three good meals a day, and don't snack."

What you mustn't do, though, is deny your child food. "The concept of dieting for children is completely inappropriate except in the most extreme cases," says Jebb. Instead, the trick is to make changes without your child realising. "Forget to fill up the sweet drawer," suggests Gateley, "and fill up the fruit bowl instead."

You also have to get your child to exercise. It's just as important as eating right and it's something most British children don't get anywhere near enough of. Less than 5% now walk or cycle to school compared with more than 80% 20 years ago. Most spend their leisure hours slumped in front of the TV or computer screen. One US study showed a 2% increase in child obesity for each extra hour of telly watched.

Setting a good example works here, too. A study of children aged four to seven found that the ones with active parents were seven times more energetic than their lazy-parented friends. "Kids need to associate activity with fun," says Gateley. "Obesity doesn't develop overnight," says Jebb, "and you can't treat it overnight either." Indeed, you shouldn't even be thinking in terms of weight loss for your child. "All you have to do is stop them gaining any more weight."