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Chafing around the area of the bottom band of sports bras is a problem for any woman during a long workout or competition, regardless of her cup size.
For women who run marathons, chafing is almost inevitable if you have a poorly fitted bra. If you are moving continuously for several hours, something is bound to start rubbing somewhere. But seamless sports bras have made a world of difference. As a moderate, 34B marathon runner, I've found that seamless bras essentially don't chafe at all, and they are so lightweight you feel like you're (almost) not wearing them. Most seamless bras come in a racerback style so there's no strap slippage, which could also cause chafing during a marathon.
But don't run your marathon in a brand-new bra, regardless of the style. Even if it's your favorite, it's best to wear it for a training run and wash it once before a competition, so you are sure that it fits well.