I ordered a s/m and received a medium. fits perfectly. 32 C, 5'2, 100 lbs, and I usually buy swimsuits from aerie and pink. at aerie I'm a small top & bottom, at pink I'm a small top & extra small bottom.
Love this suit so much!! I bought the blue one first and when I got it I fell in love so I had to order the other color. (Check my other review for pictures and details)
It's also really nice cause you can mix match with a black swim suit (black top with bottoms / black bottomed with top)
So really you can get 2 suits out of it.