Another thing that can change over time is that the shape of your breasts change. Sadly, gravity does take a toll and most women find their breasts hang lower and are not as "perky." You may find you want different qualities in a bra, such as side support, back support, wider straps, underwires or no underwires, etc.
The best guide is how well the bra seems to fit you, and if that is changing, you probably need to try on both new sizes, and new styles.
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