A photograph of Keira Knightley is always an arresting sight, but this time it was not because of her stunningly beautiful face and nor was it because she was exposing acres of flesh. Quite the opposite: it was her waist everyone was looking at, and a covered-up waist at that. Knightley was at a Chanel fashion show in Paris (picture right), working a dress that, to the untrained eye, was cut in half. Her 23-inch (58cm) waist was the star of the show. The repercussions for the rest of us are scarier than you might think.

For my generation, waists are pretty much uncharted waters. I have no idea what size my waist is and I doubt it’s ever been measured. Our mothers’ generation knew as a matter of course what their vital statistics were. My granny had whalebone corsets and would have spent her life in strictly tailored clothes. I, by contrast, bought my first properly fitted dress only very recently, one that nips in at the waist and makes the wearer stand up straight. It felt so odd that I thought it was the wrong size. It took a friend to point out that it wasn’t: we’ve just never worn clothes like this before. And I’m not the only one: in a highly unscientific poll of my 30- and 40-something girlfriends yesterday, not a single one knew what size her waist was. Not one.

They all knew what size jeans they wear and laboured under the misapprehension that this was their waist size, but it isn’t necessarily. Jeans sizing varies as much between brands and stores as any other item of clothing. One friend is a size 26 in Topshop, but a 32 in H&M. That none of us knows our waist size, or has ever thought about our waists when we’re getting dressed, is surely telling. For us, apparently, ignorance is bliss. Men, on the other hand, know what size their waists are but are in denial about it. According to a study by the British Heart Foundation, the average British man thought his waist was two inches smaller than it actually is, at 35in instead of the average 37in.

Dr John Briffa, author of Waist Disposal, a men’s weight-loss manual, says: “For men there is a general recognition that weight sits around the middle, and that’s where they want to lose it so they don’t look like their dads. They are more aware of their waistlines for three reasons: the waist is where the weight tends to go on; men are focused on the aesthetic in a way they weren’t 20 years ago; and there is an increasing recognition that weight around the midriff is most closely linked to early death.”

For women the picture is somewhat different. In 1951, the average British woman had a 27-and-a-half-inch waist. Today, it’s 34-inch, 3-inch bigger than doctors say is healthy. The really bad news is that our busts and hips aren’t expanding at the same rate, or in proportion to our waists. Basically, we’re getting rounder.

The reason is simple: we eat more (2,100 calories a day compared with 1,800 in the 1950s) and move around less. A 2011 survey found that the average American burnt 150 fewer calories a day than their parents did. In 1965, American women spent nearly 26 hours a week cleaning, cooking and doing laundry, but by 2010, that had dropped to just over 13 hours. Clearly, that’s a huge leap forward in freeing up our time, but it’s also part of the reason why we’re fatter than they were.

“A small waist has been desirable for most of fashion history in Western society,” says Katy Werlin, a fashion historian. “But today, the ideal body type has changed drastically: now it’s stick-thin women like Keira Knightley who aren’t very curvy who are considered ideal. Traditionally, a small waist was admired because it emphasised the roundness of the breasts and hips. The idea of a small waist has stuck around: look at the gowns worn to the Oscars. Most were form-fitting, classic and emphasised the feminine form.”

Men’s waist sizes have also increased, but unlike women their fat distribution has remained pretty much the same.

“It’s no wonder our waistlines are getting bigger,” says Rhiannon Lambert, a nutritionist at the Food Doctor clinic in Central London. “We eat the wrong things and are more sedentary. Today’s food is processed, full of salt, sugar and additives to extend shelf life, and those are not easily digested by the body. It’s healthier to have a leaner waist, but even in slimmer people the problem can be fat stored around the organs. We eat too much white bread, sugar and salt, and the calories from those and from alcohol tend to get stored around the waist. The solution is common sense: more fruit and vegetables and good protein — don’t buy breaded chicken, buy chicken fillet instead.”

Ironically, while we’re getting fatter, models and celebrities are getting thinner, although as ever these things are relative. Marilyn Monroe was said to be a size 16, but not all size 16s are created equal. Until the Eighties, the US Department of Commerce dictated the measurements. Once they stopped, manufacturers were free to flatter their customers and say an item was whatever size they liked.

In current sizes, it’s been estimated that Monroe would be a size 8. Not so plus-size after all. Today it’s calculated that only 8 per cent of women have a true hourglass figure like hers, with bust and hips of equal proportion and a narrow waist.

Whatever Monroe’s hourglass dimensions may have measured, the allure of the waist is hardly new. In the 19th century, dressmakers’ guides actually specified dimensions for the perfect female form: a girl’s waist should be twice the circumference of her neck, which, in turn, should be twice the circumference of her wrist.

What you were supposed to do about the circumference of your neck and wrists, if they were found wanting, I’m not entirely sure. Kill yourself, possibly, or at least resign yourself to a lifetime of spinsterhood.

Whether anyone ever bothered about their neck and wrist size I very much doubt, although it’s a relief to know that fashion imposing unrealistic demands on the female form is not new. Either way, I will not be wearing Keira Knightley’s outfit any time soon, but that’s less to do with my waist and more to do with cropped tops being disastrous on big busts. That, fortunately, is a whole other story.