Shoulder width is a key here, and a kid 12 is not a good fit for a woman with 34B chesties. Men/boys stuff runs wider in the shoulder and narrower in the waist, proportionally, than women's stuff, which also runs to narrower and higher armholes. If you have ladylike narrow and/or sloping shoulders and a waist a women's 10 is probably A Choice; if you want more freedom around the arms and shoulders a men's XS will probably do the biz.