If only buying a bra was as easy as buying a t-shirt. You know, so once you reach a certain size and shape, you can reasonably expect to fit into that size time and time again (the exception to this of course being pregnancy, in which case everything you think you know about size goes out the window).
Well, here's the thing with bras. They don't seem to play by the same rules as other clothes.
Bra sizes can change seemingly as often as the weather and, it seems, without much good reason. After all, how many times have you gone into a store swearing you're a 10C only to find the sizing is all wrong once you get home?
"This is why women should always try on bras before they buy them, every single time," professional fitter for KISSKILL, Shiori Pike, told HuffPost Australia.
"Even if you're a size 12D in one bra, that doesn't mean you're then safe to go and grab a dozen other 12Ds off the rack and assume they'll all fit the same."
According to Pike, the reasoning behind this comes to a combination of brands, fabric and a misunderstanding of how bra sizes work.
"So for starters, it's like with fashion. In different brands, your sizing can be quite different," Pike said. "I'm sure lots of women will know it's possible to be a size 12 in one brand, a size 14 in another and a size 10 in yet another.
"But even though lots of women know that, when it comes to bras, if they're a standard 12DD in one brand they automatically presume they're a 12DD across the board, when that simply isn't the case."
Another factor that comes into play is the shaping of the bra. In other words, it's very common for a woman to be one size in a push-up but a completely different size in a t-shirt bra.
"I know for me personally, I might be a 10E in a half cup with a bit of a stretch back, but if the back was more rigid with more of a push up, I might be more like a 12D," Pike said.
"It really depends on the shape of the bra and the fabric used. If a bra is stretchy in the back, you might find you are slightly smaller in the band. Whereas if it's tighter in the back or double lined, you might need to go up a size."
Interestingly, while Pike says it is completely normal for women to differ in sizes, women also tend to be very protective about that 'their' size is.
"A lot of people also think the bra sizing is the same as a dress sizing, but it's not," Pike said. "I hear a lot of 'but I can't be a 12, I'm an only an 8 dress'. Women get quite worried.
"In reality [bra sizing] is more based on your rib cage and positioning. It's not like fashion, it's completely different to that, and women should never freak out when it comes to being a slightly different size. Different brands do vary so much."
In terms of how to tell whether your bra is fitting properly, Pike says there are a couple of things to keep an eye out for.
"Always look for the gape or whether it's cutting into the top of the breast," she said.
"Pay attention to how the wire fits, as [it can differ] across different shapes. For instance with a push up, the wire will sit inward a little more.
"Make sure the back is sitting straight across, and that you are doing it up on the last or the middle hook for a true fit, not the tightest.
"Sometimes in bras you'll go for a sister size. This means you might normally be a 10D but, if there is a firmer band, the 12C will fit around the same. A good rule of thumb for this is you'll tend to go up [a size] in the band, and down in the cup."
As for Pike's final advice? Always, always try before you buy.
"If you are buying a dress, and spending the money on it, you're going to try it on," she said. "The same should be applied to lingerie."