"We like to say that they’re sisters, not twins," Cohen says. "Very few women have perfect and symmetrical breasts," she explains. Altman's own experience in bra fitting confirms this: "I would say at least 75% of the women I see have one boob that’s bigger than the other, and they get really self-conscious.” Across the board, at least 25% of adult women have asymmetrical breasts. The difference can be very, very subtle, or up to a full cup size in some women, says Cohen. You also have to keep in mind that the size of your breasts fluctuates throughout the month — depending on your period, weight gain or loss, and other lifestyle changes, reminds Bernadette, which makes a difference in how the bra fits. "I like to do it around life events," like pregnancies, breastfeeding, and new routines, says Cohen, "because you know your body more than anybody else."What this means is that, while identifying that magic number and letter is helpful, the quest for the perfect bra doesn't (and shouldn't) end there. It's important to find the right fit — which means trying on different styles, and maybe going up or down a size, depending on how it feels. Altman likens it to shopping for denim: You may be one number for one specific style at one brand, but that may change when you switch from skinny to boyfriend jeans, or vice versa. "It’s almost like the sizes are comparable, but not exact," she says.Bernadette recommends going in "for a fitting once a year," as a sort of temperature check to make sure your bra fits comfortably (and correctly). Town Shop doesn't put bras on display for that reason: You have to see a fitter to get your hands on the style you want, which ensures you find the right size. "I think the biggest mistake that women make is saying, 'I’m a 34C, and I’ve always been a 34C,'” shares Cohen. When she hears that, she thinks: "Your body changes on average six times in your life, so chances are, unless you’ve recently been fitted, you’re going to be a different size — and that’s okay." She stresses that size is what's key, regardless of number.