I spent some time last weekend blow-drying my hair in a communal changing room at one of those spas that attracts hen parties and mother-daughter bonding combos. In the mirror, safely shrouded by a long fringe, I examined the room's spectrum of breasts. I saw one woman pick each of hers up and apply a sift of talc beneath, then replace them as if laying patio slabs. Another corralled hers into a front-loading bra, hauling them in like she was landing a couple of marlin. A third, gawky in a rolled-down Speedo swimsuit, stretched to reach for her bag in a high locker, and her breasts disappeared altogether, leaving just a nipple as a giveaway, pinned to her torso like a badge. I sat there on a leatherette stool, my own chest trapped coyly in a tight towel, thinking that this unscientific survey revealed much about what the nation really keeps in the privacy and warmth of its brassieres: squashy cushions, south-bound dugs, breasts as inconsequential as a sneeze, as impressive as an ocean-going liner. All kinds. All sorts. And each so far removed from what Naomi Wolf a decade ago dubbed the 'Official Breast' as to dwell upon another planet.

It would be easy to forget what a real bosom looks like these days, what really lurks under the cotton shirt of 21st-century woman. The public breasts that populate our culture bear little relation to what's going on under there. I caught sight of a bizarre picture the other day - Victoria Beckham was being escorted by David to her birthday party in LA; at first glance, she appeared to be wearing her breasts as earrings. They were way too far up her body, as if they'd crawled north to get a proper look at her face. What a strange place to keep tits, I thought, recalling my own, which always seem to be more interested in my stomach than my chin.

Actually, I've always rather liked my chest, ever since it arrived in 1979, delivered overnight like a couple of FedEx parcels. In the intervening years, I have regularly got it out for parties, as if laying on a buffet. I like its jaunty, conversational appeal, the fact that it generally behaves impeccably in public, even if in private it can be a bit of a drag. Anyone blessed with more than a B-cup will recognise the many moments when you'd happily lob them over the nearest fence. In yoga classes, for instance, the risk of my being suffocated is high; lay on my side and they flop on top of each other like sleeping puppies; and, if I'm not very judicious in my choice of underwear, they can be an absolute liability on a treadmill. So, as you can see, I think a lot about my chest. Most women do: it's an important and singular bit of the anatomy - part fun-park, part albatross.

Over the past few years, though, something curious has happened to breasts. More than ever before, they've been outed, even in relatively polite company, so much so that I can't help but groan, 'Oh, do put 'em away love' as I witness the Oscars coverage or Brighton town on a warm Saturday night. Leafing through OK! the other day, I came across coverage of Tim Jefferies' wedding at Blenheim Palace, to which the cream of London society had been invited. There was Elizabeth Hurley, Elle Macpherson, Cindy Crawford - all women in their 40s, all sporting an impeccable, mad-for-it cleavage, breasts straining at silk jersey, nipples ahoy. It was a jug-fest, each more bronzed and beautiful than the last. A couple of years ago, I would have been checking out their handbags, I thought as I idly turned the pages. Instead I'm sizing up their tits.

Now that we've tired of It Bags, peachy It Breasts have rolled into the public arena as if to pep up proceedings. Anyone on a red carpet who keeps theirs under wraps looks positively prudish. Most celebrities, people who wouldn't dream of discussing their children or their religious beliefs with the hoi polloi, are quite happy to ruminate publicly upon their breasts at the merest opportunity. If it's not Keira Knightley sighing 'I would love to have tits', it's Denise van Outen, mentioning that she likes 'them to be big and round', or Nicole Richie, who admits that she isn't much taken with her new motherly bosom ('it doesn't really fit with my wardrobe' - ach, what a bitch life can be).

Meanwhile, television has gone into a boob-overdrive. Carrying on the worthy British tradition developed by Benny Hill, Diana Dors, Babs Windsor and Sam Fox, we now get Trinny and Susannah goading hapless women into unleashing their juggernaut breasts on an unsuspecting world. Or Gok Wan, endlessly congratulating his girlfriends for having really brilliant bangers, as if they won them in a spelling competition. And if we weren't glued to those wicked, pointy chests that littered the set of Mad Men, there's always Nigella's cupcake chest to ogle. Who could resist it, anchored there in its plunge-neck sweater, excusing the rounded belly and widening hips by its sheer ooomph? ('There's nothing like a huge pair of breasts to give you a tiny waist,' confides my friend Lizzy, 'I've got my mother's chest - a double-D - and everyone thinks I'm really nipped-in. I'm actually built like a box, but the breasts do the business.') That's it, you see: the breasts do the business. How peculiar that a great set - whether natural or, more likely, engineered - seems to have become, with little reverence for the march of feminism, the pinnacle of womanly achievement in the 21st century.

There has always been a chest of the day, of course, something to gauge oneself against, to aspire to. History tells of the flattened flapper, the matronly bulk of the war years, the twin rockets of the Fifties. Marilyn Monroe's breasts were apparently held solidly in place, 24 hours a day, by the 'Bullet Bra', which may well have added to her general disillusionment with life. Breasts, you may recall, shrank in the Sixties, and were then left pretty much to their own devices in the free-love years of the early Seventies, when bras were burned and boobs went all romantic and soft-focus. Then, ta-da!, out they came in the Nineties, housed like a couple of scoops of vanilla Häagen-Dazs in the all-new, woo-hoo Wonderbra ... . And so it goes on - in, out, up, down, high, low - the ideal bouncing all over the place, the idea constantly in transit, going somewhere but never quite arriving. Over the years, women have been exhorted to do so many tricks with our tits that we're practically juggling. Flatten them to look your best in a vest. Fatten them up to fill out a dress. Plunge. Push up. Parade. Reveal. Conceal. Inflate to score in a bar. Deflate to succeed in a boardroom. Disguise them to look clever. Unveil them to look hot ...

Today's Official Breast, though, is harder to harness, being neither one thing nor the other, a tale of two extremes. On the one hand, the Fashion Tit remains more of a torso with nipples that might have been drawn on with a felt-tip pen- a fleeting thought between ribcage and clavicle. According to Karen Diamond, director of the Models 1 agency, 'Tall, slim and small-breasted remains the ideal shape among catwalk girls. The more commercial girls do need to be at least a 34B for high-street work, where companies get complaints from customers if their models are too bony - but the catwalk girls need to be pretty flat.'

There are feted exceptions to this rule (Gisele bounces to mind), but, more often than not, catwalk models can be painted in a single brush stroke from neck to knee, their greatest protuberance being a pair of aggressive hip bones. The reason for this lack has long been the subject of debate in lofty fashion circles. In purist aesthetic terms, it is argued, clothes tend to hang more fluidly on a frame without a rack. Look, for instance, at how Erin O'Connor's figure goes on and on for miles and miles, your eye arriving finally at that distant black bob having never been troubled by the annoying hike of a breast. She may look like a phone mast but, in fashion terms, this bulge-free whip of not-quite-womanhood is elegant because it is androgynous, android. Think of L'Wren Scott, Agyness Deyn, Nicole Kidman. Not sexy, exactly, but they sure can work a wicked black dress.

More prosaically, this boyish, boobless template well suits many a male fashion designer, who might view proper wobbly pannacotta breasts as something of a threat. They're not big into darts, these men, nor underwiring, nor any of the canny geometry required to adequately accommodate these objectionable orbs. Over the years, then, this has become the fashion paradigm. Through hippie dressing, via heroin chic, 'mannish' tailoring and the boho look, the breast is erased from the picture, an adolescent bud installed in its place. The exceptions to this rule - the Jean Paul Gaultiers and Dolce & Gabbanas of this world - grasp wildly at the other extreme, generally arriving at a conical, warrior-like breast, a cartoon as far removed from the reality of womanhood as its washboard cousin.

Clearly, neither version - the bazooka babe nor the models with a question mark where their chest ought to be - bears much relation to the majority of real women, the ones who lift and separate, who sag and sigh and subject their own homely bosoms to the ignominy of tit-tape or chicken fillets. Ask any woman how she feels about her own breasts and you'll probably get a bit of a moan: we want what we haven't got, which is why the business of augmentation and reduction has proliferated in the past decade or so.

This brings us neatly to the Other Official Breast, the fashion alternative to flat-pack furniture. While the fashion industry has long laboured to disguise or excise the breast, some time ago the beauty industry started to seek it out, put it on a shelf and charge people to roll up and buy their own.

Since the turn of the millennium, a new type of tit has been wildly popular. These are the super mammaries, designed by computer, installed by technician, round, firm, buoyant, and yours for the price of a holiday in the Algarve. What's really weird is that so many of these 21st-century breasts - the ones we see on Posh Spice and co - are often so steadfastly phoney, so clearly grafted on, that they're verging on hilarious. Guardian writer Libby Brooks calls these uber-boobs a 'cut-and-paste womanhood, a parody of vapid and vanquished sexuality ... the logical conclusion of the relentless commodification of the female form ...'. Jay McInerney called them 'party tits'.

The look - tarty, taut, permanently erect - has become so commonplace that 'surgeon to the stars' Patrick Mallucci has arrived at a mathematical equation to describe the perfect breast: 'If you look at someone who has attractive breasts,' he says, 'then draw a line horizontally through the nipple, you will see there is more breast below the line than above it. The ideal is a 45 to 55 per cent proportion - ie, the nipple is pointing slightly skyward.' Welcome to a world in which women are increasingly interested in looking like a digital avatar, one part Lara Croft to two parts tennis ball.

Brooks is right that, with the addition of these artificial helium balloons, the paradigmatic female figure becomes grossly distorted, as ludicrous in its own way as the bustled behind of Victorian times or the bound feet of the Chinese during the Middle Ages. At the extreme, you find odd fabrications of femininity such as Ashley Bond, Alicia Douvall, Jodie Marsh or the pre-jungle Jordan, women with valves in their armpits allowing their breasts to be inflated at will. Unsurprisingly, Germaine Greer, in her book The Whole Woman, likened breast augmentation to African genital mutilation but, over the past decade, there appears to have been a shame drain in this particular area, with footballers' wives and TV totty inviting us all to cop an optical feel and a horde of normal women deciding to save up and follow suit. They can't all be mad. There is something provocative enough about 'perfect breasts' to make otherwise sane women agree to having their nipples sliced off and reattached in a more promising position. Why? For men? This seems unlikely - since, should you engage them on the topic, men tend to be grateful for small mercies. As Phil Hilton, ex-editor of lad magazine Nuts, once put it, 'Men think all breasts are good and are delighted to have access to any at all. The idea that they are connoisseurs is inaccurate.' One friend of mine, T, is adamant that her recent surgery was expressly to please herself and allow her for the first time to enjoy wearing low-cut tops. 'It's relatively cheap, you recover quickly, there's barely a scar and now I've got a bikini body,' she says. 'Get off my back with this genital mutilation crap - it's my body and I'll do what I like with it.' Comments from other women who've undergone surgical procedures seem to back this up: 'It was really one dress that suddenly made me wish I had something more ample up front,' says one, who swapped her A-cup for a C. Kelly Rowland, formerly a third of Destiny's Child, says she had a boob job because 'I was sick of not fitting tops and I had a hot one I wanted to fill out.'

Despite all the obvious downsides - the horrifying prospect of leaky implants, the idea that part of your body should be subject to guarantee, like a dishwasher - despite all of this, having one's breasts tarted up is, increasingly, no big deal, about as emotionally demanding as ordering a new conservatory. For some, it seems that the trophy chest is a bit like having a Jacuzzi on the deck or a Porsche with cream leather seats (as one man, whose wife had installed implants, put it: 'Of course you can tell they've been done, but that's part of it. I really appreciated that she'd made the effort.') Without wishing to sound like a boob snob, the upshot is arriviste, it's nouveau. It suggests money and style, in the way that Versace or Burberry suggest money and style.

Which is why, increasingly, this particular look seems old-fashioned. Like a deep tan, or matching Vuitton luggage, a pair of obviously man-made breasts is starting to look ever more peculiar. The sight of Victoria Beckham's bristols popping up out of her tiny dresses like a pair of gophers, well, it's all so desperate isn't it? So 2006? The fact that Jordan - a woman who did so much to put super-mammaries on the map - has dropped not only her stage name and her blonde extensions, but several pounds of breast meat, too (word has it she stuck her implants on eBay), is evidence enough that mega-jugs are on their way out.

Indeed, in the past year, 4,000 women in the US have had their implants removed. According to the Sun, an undoubted expert in the field, 'Fewer women want boob jobs and other cosmetic surgery in a backlash against trout pouts and the Jordan effect. According to market analysts Key Note, only 16 per cent would now consider going under the knife, compared with 20 per cent in 2006.' Quite what initiated this change of heart is hard to discern. Was it the sight of Jodie Marsh struggling under the weight of her 11 GCSEs and twin inflatables? The news that glamour model Alicia Douvall's daughter asked for breast surgery for her 13th birthday? (Douvall - who has herself had a dozen boob jobs - agreed that Georgia could have the operation, but not until she was 16. 'I'm happy for her to have a boob job. It will give her a career,' said Douvall. She also told Closer magazine she had changed Georgia's name to Destiny to enhance the girl's chances of success.)

Whatever the catalyst, it's clear that the bubble is bursting for phoney boobs. Even the feted Dr Robert Rey, US breast surgeon to the stars, seemed downbeat recently about his chosen profession: 'Inner beauty,' he said gloomily, 'is disappearing in this country.' As the new-look, nearly-real Katie Price says, albeit through pillow-lips, 'You can't beat natural beauty.'

Well, actually, you can. Today, anyone who's a bit disappointed with the volume and perkiness of their bust can opt for an injection that promises to revive the droop of tired old tits. Now - as the beauty world's early adopters will already know - you can micro-manage your chest with Macrolane, an injection that lasts 18 months, costs as much as conventional implants, and involves hyaluronic acid being fed deep into the breast via 'a thick needle similar to a knitting needle'. Irresistible, eh? Well, yet again, women are plumping for it in droves. The point about this procedure, unlike its more gaudy forebears, is that it's for personal gratification, not for the pleasure of others - since a spectator would be hard-pressed to notice the difference after the event.

And so, the Official Breast is on the move again. Perhaps it will come to rest at a happy medium, lodged somewhere between those giant turbo tits chosen from a laminated catalogue and the flattened nothingness of fashion. It may revert to something more natural, as it did in the Seventies, when a soft, bra-less, real-time bosom briefly made an appearance, housed in chiffon, charming, feminine, approachable, familiar and doing swirly dances on Top of the Pops. Scarlett Johansson's got them already. Kirsten Dunst, too. If anything, these softer, fuller chests are more akin to what you keep stashed in your own bra. With luck, women will start to want the real deal, ones you really encounter - a lovely handful, with some weight and perhaps 'a bit of droop' as art critic Waldemar Januszczak says of proper boobs. A real woman's breasts, he notes, when released from her bra, 'naturally sag and separate'. Could this be the next Official Breast? If so, I'm in - and, if the communal changing room is anything to go by, I bet you are, too.

· Mimi Spencer is a columnist for You magazine