Things never to say to a teenage girl: "My, you've put on a few pounds," or a variation: "Have you put on a bit of weight?" or even, unforgivably: "You're getting too fat".

Astonishingly, while some parents tread cautiously around the subject of teenage weight gain, terrified of saying anything that might trigger an eating disorder, there are still adults who will walk into this minefield with hobnailed boots on.

One sensitive 14-year-old refused to go back to her family's GP after he said, jovially: "You're getting a bit chubby". Another girl didn't eat anything for three weeks after her mother said: "I think you're getting a bit fat".

Here is the news. Teenage girls are sensitive about their weight. In a culture that has made everyone, but girls and women in particular, fat-conscious, it seems that many, perhaps most, girls are watching their weight - no matter how thin they are.

"I know I can be a size six if I diet," says one healthy, slender, girl of 5'5". "Ugh, nine stone, I've never been nine stone," wails another. Word filters back: "So-and-so is on a really serious diet, she only drinks water for lunch and eats tuna salads. I wish I had her willpower." And tragically, of course, every year you hear of the girl whose obsession with weight has crossed the line into a serious eating disorder.

Parents may rail against the insanity of a culture which prefers Kate Moss over Kate Winslet as the ideal body image, and has made nearly all women, as well as girls, dissatisfied with the way they look. In practice, how do they help a daughter who is worried about her weight? Or help a teenager who is sensitive about excess pounds, but who won't talk about it and "comfort eats" because she's upset? Crucially, how can you tell when a child might be on the point of crossing the line, of tipping into anorexia or bulimia?

Senior psychologists Andrew Conway and Marie Murray both point to the bitter unfairness of a culture that makes girls as young as 11 and 12 hyper-conscious about their looks at exactly the point when so many of them explode into puberty with puppy fat that will almost certainly be shed by late adolescence if they are eating half-way sensibly.

The truth that human beings come in all shapes and sizes and that it is obviously ridiculous for all women to aspire to a "norm" - tall, stick-insect thin - is nowhere more obvious than in a class of 13- and 14year-old girls. Some still look as if they are eight or nine, drowned in a secondary school uniform, others could pass for 20 or more, with 36B busts, and child-bearing hips. And yet of course this is the age where they all desperately want to look the same.

If your teenager is a little on the chubby side, you can be pretty sure that she is sensitive about her weight, says Conway - teenage culture will make it nearly impossible for her not to be. But should you bring up the subject if she won't talk about it? If she does want to diet, how can you ensure that she eats enough to stay healthy?

Cara Gray, dietician with the Midlands Health Board, says that her big worry is that teenage girls don't get enough calcium, because they turn away from dairy products in the quest for thinness. Many also don't get enough iron, since they refuse to eat red meat, its most easily available source. Like all health educators, she is anti-diets, promoting healthy eating instead - and says that parents should encourage their daughters to eat plenty of diet yogurts, and drink low-fat milk to make sure they get their calcium at a critical time for young bone development.

Senior psychologist Marie Murray also points out that healthy attitudes to food are created long before adolescence: "There should be no arguments at mealtimes from the outset, when children are very young, so that meals are not associated with stress - food should not become part of a power struggle. You should allow children a reasonable choice of food, especially teenagers." And of course, you should provide plenty of healthy snacks, like fruits and yogurts, providing bars and crisps only in moderation. SAYS Murray: "You should never remark on a child's over-weight", and adds that there is no place for a parent to comment on "cosmetic" or "visual" overweight. But both Conway and Murray say parents shouldn't ignore gross overweight if it is a problem, either medically or psychologically, and should seek medical advice as you would if your child had any other health problem. Ideally, though, wait for the teenager to bring up the topic herself: it will come up, says Murray, perhaps in relation to buying clothes.

If a child is slightly overweight, Murray says that you could say: "You need to get more fit," and propose some shared activity; if a child expresses concern about her looks you should reassure her: "You're lovely, but if that's the way you feel, why don't we go walking."

Conway suggests having a general discussion about family eating habits, perhaps agreeing that everyone needs to eat more healthy food. And, says Murray: "You can quietly prepare less fattening foods, attending to their weight without discussing it. Make healthy snacks - fruit, for example - visible. Drop them in a peeled orange when they're watching TV."

Both see no problem in perhaps letting your teenager accompany you to meetings of an organisation like Weight Watchers, or enrolling themselves - Weight Watchers is particularly conscious of teenager's dietary needs.

The ideal, as Cara Gray says, is not to think "diet". The problem, of course, in our junk food culture is that normal healthy eating nearly is dieting, as one teenager wailed recently to her mother. And many teenagers are caught in this dilemma. As Conway says, ask young teenagers what their favourite meals are and they'll say pizzas, Macdonalds and Chinese.

But Cara Gray believes that parents and girls shouldn't worry too much about this - if you choose a thin crust pizza, and perhaps grate a little extra cheese on it, it constitutes a reasonably healthy meal. And says Murray "I wouldn't worry too much about them eating crisps and so on - they're growing, they're hungry. I'd worry more about a parents saying negative things like, `don't eat that, you'll put on weight'."

What if you're worried that your child is heading for a serious eating disorder? Deception is one indicator, says Murray - a girl wearing clothes that hide how thin she is, or lying about having eaten. Another sign is shuffling food around a plate, making neat bundles that make it look as if food has been eaten - signs of anxiety generally. Cooking food for the family, but not eating it. Another sign, says Andrew Conway, is a girl dictating what can and can't be in the family fridge - because she can't bear to look at meat or other "fattening" foods.

Conway, Murray and Cara Gray all point out that eating disorders are not - ironically - about food at all, and are usually to do with deeper psychological problems about issues of control. But as with alcoholism, people who diet obsessively are running a risk.

But all agree - if a parent suspects that a child is heading in that direction, you should go immediately to your GP. There is no room for pussyfooting around the issue.