Are you constantly pulling up your bra straps? Do your shoulders fall forward? Do your bosoms bounce when you walk, despite your new balcony bra with lace trim? The chances are you are among the majority of women who have never had a proper bra fitting, and whose posture and back health are compromised as a result.

Attitudes about bra fit and function vary by generation. Younger women are more likely to find that their bra doesn't fit properly because, in the good old days, pubescent girls were almost always fitted for bras. The idea of being fitted for a bra when I was 13 would have been gross, but everything looks different after my bra epiphany, which happened in Night Owls on the Fulham Road, in south-west London.

"Your bra doesn't fit properly!" the lady at the cash register shouted, as I checked out the pink flannel pyjamas. I pretended not to notice. "I bet you've never had a bra fitting in your life!" continued the voice. She marched over. I was wearing an expensive underwired bra, designed to give "lift". I had tried it on in the shop, but that was not, I was told, a proper fitting. "I bet your back strap's half way up your shoulder blades. Look at your posture! Your bust line is sinking!" My ear lobes, she said, were almost attached to my shoulders. I'd spent years compromising my breast and back health, apparently, and my bra was a load of "shite". I told her I'd always been a 36C. "Rubbish," she snorted. "You're a 30- or 32E."

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This was a shock; I'd only come in for pyjamas. As chairman of the PTA, I did not want an E cup. It sounded huge, like Jordan.

All this is not just a matter of vanity. Last week, the British Chiropractic Association (BCA) warned that wearing the wrong bra size can lead to a number of problems, including back pain, restricted breathing, abrasions, breast pain and poor posture. The problems are even more acute in large-breasted women. "Bras are like suspension bridges," says the BCA's Tim Hutchful. "You need a well-engineered bra so your shoulders don't end up doing all the work. Bras that don't fit will affect the shoulders and chest, and will almost certainly cause back pain as you get older."

The woman prodding, pulling and hoiking introduced herself as Yolanda Ktori. A passionate advocate of corsetry, she has seen the profession clobbered by our smash-and-grab society. "Women buy bras off the shelf at M&S [though it does offer a fitting service], and don't have a clue what size they are. Some stores offer fittings, but most women don't bother." She pushed my shoulders down and pulled my blades back, showing me where the strap should sit: on the meaty part of the shoulder towards the neckline, locking in with the shoulders and streamlining down the back to stop slipping. "It's like the reins on a horse. You pull everything back, not forward," she says.

The corsetry experts at Rigby & Peller estimate that 80 per cent of women that come through their doors are wearing the wrong-sized bra, and also stress that badly fitting bras can lead to back problems and bad posture. The most common bra solecisms are cups that are too small and bands that are loose around the back causing the shoulders to carry the weight of the bosoms, instead of the back. A bra must be firm (not tight) around the back and support the weight of the breasts in the mid to lower back. Rigby & Peller says the back band should be level with the underwire (Ktori says an inch-and-a-half lower).

Jon-Morton Bell, an osteopath, agrees that properly fitting bras are key to back health. "Ladies have to do a balancing act with gravity," he says. "Proper support for the breasts has a huge impact on back health. The best place to support the breasts is through the lumbar (lower back), but often, women take the strain through the thoracic (around the ribcage), which can cause a curved back. If a woman is bending forward because of insufficient breast support, the trapezius overstretches and causes headaches. All nerve roots come from the back; stomach upsets and fatigue are common by-products of bad back health. If ladies have a proper bra-fitting, back problems are often resolved."

There are even those who have claimed that there is a link between underwired bras and breast cancer. In their book Dressed to Kill, Sydney Ross Singer and Soma Grismaijer claim that the external pressure of the bra constricts the lymphatic vessels and prevents proper draining of the breast tissue, leading to fluid accumulation in the breast. Western rates of the disease are 10 times those of developing countries because, they tell us, women in the latter tend not to wear bras, let alone cleavage-heaving cross-your heart-and-hope-to-die wonder bras. They urge women to abandon bras and embrace freedom, but don't substantiate their claims with any supportive comparisons in diet, lifestyle or genetic history.

The metal in underwired bras is unpopular with acupuncturists as it crosses the body's meridians and blocks the flow of chi, they say, which can cause energy to stagnate. But breast cancer? Lynn Daly, from Cancer Research UK, says: "You would need to wear a bra that was painfully and unbearably tight to have any effect on your lymphatic system, but constriction or applying pressure to an area of the body does not cause normal cells to become cancerous."

"It's not rocket science," says Yolanda Ktori. "Saggy bosoms are bad for your back. Get a bra that fits properly. You wouldn't wear shoes that are too tight, would you? Women who have their bra fitted properly tell me it's changed their lives."

The wrong bra? How you can tell

Kate Horrell, fitting expert for the online lingerie shop figleaves.com, recommends looking closely in the mirror for the telltale signs that you're wearing the wrong bra size.

The underband is riding up

Lift up your arms to see whether the bra is tight enough. The underband should fit firmly against the body so that it does not slide about during normal activity.

The shoulder straps are digging in

The underband of a bra provides the majority (80 per cent) of support for the breasts, with the straps providing just 20 per cent. If the bra straps are digging in, it could be because the underband is too loose and you are over-adjusting the straps to feel supported. When you do this, the straps pull the bra up at the back, another sign that the band is too loose.

The centre fold is lifting away from the body

The centre front of the bra should lie flat against the body. If it doesn't, the cups could be too small.

The back band is overstretched

If your straps are too far apart at the back (they should be parallel), it may be that your bra band is too small and so is overstretching.