they depend on the band size (and if I am not mistaken often even a few other parameters) of the woman.
Another factor is measurements aren't consistent between companies. This is true for more than just bras (New Balance has me a whole shoe size smaller than Nike, for one example), but female clothing in general is the worst victim of this disparity. Then there's the factor of multiple fastening points, since few women measure
exactly to their particular band size, that prove even if a bra size is supplied, it's not the exact measurement it's presumed to be. And finally, we have to remember human error. It's present in almost everything, but sometimes it isn't abundantly apparent. A simple example is that some garments that are marketed as the same size by the same manufacturer have a slight difference in size due to someone loading the machine slightly off-center, the cutting tool having bent or slipped, or some other extremely minute change. Most times it's irrelevant - the panties suffer for this more often than the bra - but it can be a hassle at any size for any garment. The short version is every bra is about as unique as the breasts they support, but the difference is much more subdued for the former, and therefore useful only as a buying guide. And I've yet to talk about mislabeling.
One additional problem with using a bra measurement is the general ignorance of the subject. One of the most infuriating moments of my interactive fiction career came several years ago when I specifically gave a female character a bra size of 37DD. The reasons for this tiresome effort and why it ended in such a state are:
1.) This woman was tall and fit. I scoured the internet looking for women with the same approximate height and build posting their measurements for some health question, as well as read several for-women magazines just to see what a realistic band size would be for this character. No one cared, and on a few occasions it was ignored completely. As an aside, this was a frequent complaint even without this specific character. My work towards realism went, and continues to go, unappreciated.
2.) The reason I was so particular about this bra size was because everyone knows DD are very big, and my searches revealed bras are always made in even band sizes. By extension of the latter, I found several women with an odd band size (most of whom double checked their measurements to confirm the odd number) complaining about the difficulty of finding a well-fitted bra. I specifically wanted this bra size not just for the realism, but for the hazards. The odd number and large cups were supposed to create room for a small quantity of bras and frequent snapping, thereby generating lots of jiggling, shopping, and laundry. Including the bra size was my way of conveying this detail without explicitly stating it. No one caught on, because I was the only one who invested enough time to learn this, and apparently the only one who would raise an eyebrow at this foreign number and take to the internet before moving forward. One writer did use the "her bra is too tight" idea, but included the plot point of "her breasts have grown since she bought this older bra" instead of building off of her odd size.
If people in general were more informed, I think it would be more useful. If the numbers aren't a horrible mismatch, I know enough to create the picture, but the sheer number of failures from other writers proves I might be alone in this regard. At present, the vast majority of the audience knows only that DD is really big, and C and bigger jiggle when they're not in a bra. Trying to appeal to such an audience with bust, hip, and waist measurements, or any measurement for that matter, doesn't work. The payoff is always the same, no matter the effort behind those numbers.
My suggestion: write the numbers down on a side sheet in order to hammer out the appearance, then substitute them for words as they're transferred to the master copy. Those numbers can be extremely useful for the process, but should be omitted from the product.
