Women often find it difficult to find the correct bra size. To achieve perfect sizing consistently, a bra would have to be custom made, because a "one-size-fits-all" manufacturing process is fraught with difficulties. Breasts vary in the position on the chest, and in their diameters.
A number of stores have certified professional bra-fitters specialists. However, even bra fitters have been shown to be quite variable in their recommendations. Buying "off-the-shelf" or "online" bras is unwise if the buyer has never tried on the brand and type of bra that they are interested in buying.
Some bra manufacturers and distributors state that trying on and learning to recognize a proper fit is the best way to determine a correct bra size, much the same as with shoes. Some critics observe that measuring systems such as the one described above often lead to an incorrect size, most commonly too small in the cup, and too large in the band. For anyone, especially cup sizes larger than a D, one should get a professional fitting from the lingerie department of a clothing store or a specialty lingerie store.
Some women intentionally buy larger cups and pad them, while yet others buy smaller cups to give the appearance of being "full". Finally, the elastic properties of the band make band size highly unreliable, and in one study the label size was consistently different from the measured size. Fashion and image drive the bra market, and these factors often take precedence over comfort and function.
As already noted, there is no agreed standard across all manufacturers for measuring and specifying bra size. Obtaining the correct size is further complicated by the fact that the size and shape of a woman's breasts fluctuate during her menstrual cycle, and also with weight gain or loss. Even breathing can substantially alter the measurements. It is frequently stated, from the results of surveys, that between 70 and 100% of women are wearing incorrectly fitted bras. This may be partly due to a lack of understanding of how to correctly determine bra size; it may also be due to unusual or unexpectedly rapid growth in size brought on by pregnancy, weight gain, or medical conditions including virginal breast hypertrophy.
As breasts become larger, their shape and the distribution of the tissues within them changes, becoming ptotic and bulbous rather than conical. This makes measurements increasingly unreliable, especially for large breasts. Similarly the heavier a build the woman has, the more inaccurate the underbust measurement as the tape sinks into the flesh more easily. Finally, most women are asymmetrical (10% severely), with the left breast being larger in 62%, especially when the breasts are large.
Many of the health problems associated with bras are due to fitting problems and are discussed further below, under health problems. However, finding a comfortable fit is described as very difficult by many women, which has affected sales. Medical studies have also attested to the difficulty of getting a correct fit.