OOPS, we did it again. Britney's new underwear collection might be flying off the shelves - but the vast majority of us are still wearing the wrong size bra.

Over the past couple of years, the average UK breast size has plumped up from a 34B to a 34DD, according to research by Gossard. But 70% of women are reportedly still not wearing the correct bra size (76% if you go by Triumph's statistics); we don't seem to know what we're doing when it comes to shopping for undies.

And it's no wonder, really, when our busts yo-yo up and down so frequently. "Our boobs change for all sorts of reasons throughout our lives," explains Bravissimo's customer services manager, Louise Gardner. "Age, weight variations, pregnancy, and use of the contraceptive pill [can all alter the size of your boobs]. You may not be the same size for longer than a few months, let alone a lifetime!"

But given that we spend an average £88 a year on underwear, it's high time we picked out our perfect size.

Frequent fittings

Regular bra fittings aren't just useful for knowing how big or little your breasts are (and interestingly, Gossard report that women typically have a bigger left breast), it's also vital for keeping the rest of your body in check.

"It's so important to make sure your bra is properly fitted," explains chiropractor Rishi Loatey from the British Chiropractic Association. "This will give you as much support as possible and reduce strain on your back. Bras that don't fit will put pressure on the shoulders and chest, and may cause back pain and affect your posture."

It's recommended that you go for a fitting every six months, increasing to every three months if you're pregnant, and get re-measured if you gain or lose a significant amount of weight.

Measure up

"First, we'll check you're wearing the correct back size," says Gardner. "The strap should be firm around your body, horizontal at the back and not ride up. You should be able to run two fingers under the band."

The fitter will "check to ensure the wires sit flat between your boobs and aren't being pushed away from your body", she adds. "Your boobs should be enclosed in the cup with no wrinkling of fabric. They shouldn't bulge over the top or sides of the cups, even in a balconette or plunge bra."

DIY efforts

Professional fittings are always best, but if you don't have time, you can still check the fit at home. Always ensure the wire sits behind and underneath the breast, firstly.

"Press on the wire," says Freya's bra expert Suzanne Pentland. "If you feel soft breast tissue, then the cup is too small and the wire will dig in. If you feel bone and not softness, the wire is behind the breast and cup is correct."

If in doubt, bear in mind these pointers from Gossard lingerie expert, Joanna Van Blommestein:

  • The straps should be tight enough to give firm support.
  • The breasts should fill the cups nicely - no overspill or gaping.
  • The bra should be comfortable. Often, correctly fitting bras feel tight, but they will loosen after wash and wear.
  • A little bit of fat underneath arms is normal.
  • Be aware that black/navy bras may come up tighter than white bras - this is due to the dying process.

Where to shop

If your six months is up, book yourself an appointment. Stores offering fittings on the high street include Bravissimo (www.bravissimo.com), Debenhams (www.debenhams.com), Marks & Spencer (www.marksandspencer.com) and John Lewis (www.johnlewis.com). Women with a larger cup size, who are pregnant or who have had surgery or a mastectomy might also like to try Anita (www.anita.com). Visit the websites for your nearest store.