Bra sizes: is it all meaningless?

I'm 29 and yet I have no idea what bra size I am. A couple of years ago I thought I knew, and since then, according to what I've been told by 'professionals', I've seen myself as everything from glamour-girl buxom to on the small side. And I doubt I'm alone.

Let me explain. For most of my 20s I thought I was a 36, even a 38, B to C. Then, last year, I got measured at Selfridges and was pronounced, to my infinite surprise and delight, a 34D. 'Oh, I must buy some bras in my new size and throw the old ones away' was my first thought. Cue credit card and £100 spent, over the next couple of months, on new bras.

A few months ago a PR invited me to get fitted at Bravissimo, which specialises in bras for 'big-boobed women' and my RSVP essentially read 'Thank you but I am sure I don't qualify.' She advised me to go despite my concerns, and lo and behold the fitter laughed her head off at my 34D bra and told me I was a 32E. She told me a 34D would just not support me and the fear in my mind was of prematurely sagging bosoms in my early 30s. Maybe, at a pinch, I could also be a 34DD, she said. But 32E was 'the size' for me.

Feeling like I'd turned into Pammy Anderson within seconds, the first thing on my to-do list was to make sure I had some bras in the 'right' size. Otherwise, I would have no support! I bought more than £50 of underwear in the moments that followed alone.

And my new 32E bra looked gorgeous on. It lifted me perfectly. The problem only emerged when I tried to wear the bra for more than ten minutes. It rubbed seriously, the bones of the underwire digging in so much they left me with dark wed welts. Obedient to the fitting, I forced myself to wear the bra even though I had to go somewhere private and pull it away from my chest to relieve the welts every hour. I even sewed little cushions I made from stuffing and an old handkerchief into the bra to stop it feeling so unbearably tight, but nothing worked and after a few days I simply gave up and went back to my 34Ds and DDs.

My mother, who also had a fitting at Bravissimo, had exactly the same experience of being told she was a smaller back size and bigger cup than she had ever thought possible, buying a pretty bra, and finding it is far too uncomfortable to wear.

Then, just this week, another fitting (this time for wedding underwear - ye gods) resulted in being told I am a 36C and that a DD would be gaping on me.

All of this has left me not knowing what size I am. Obviously all bras are made differently, and all shops measure differently, but surely there should be one or two standard sizes we can assume will fit us? My mother, more cynical than me, suspects lingerie shops deliberately ask us what size we wear and then tell us we are a different size (usually flattering us but making us sound slimmer in the back and bigger in the cup) in order to make us buy new bras. I'm not sure it's that premeditated, but perhaps I'm just naive.

The only question left to answer is why do I want to know what bra size I am? Is my need to fit into a category resolving some kind of insecurity, or boosting (literally) my self-esteem? When I was told I was a 32E I felt flattered and I can't deny my body image improved. The other day when I was slapped back down to a 36C I felt like I'd just been kidding myself believing I could be any bigger than that. It's quite sad how a size can determine one's emotions.

But there is a practical need to know what bra size you are and that is when it comes to every woman's requirement to be able to shop for a bra and have an idea of what sizes to try on - otherwise a trip to M&S could take forever. My question to the sizing experts is, what are we meant to do? Who do we trust? Do we wear what is comfortable, or do we wear what is supposedly supportive?

To this end, I have made an appointment today with the Queen's corsetieres, Rigby & Peller. I understand if there is one higher authority on bra sizing, they are it. Watch this space.