I took a leap of faith ordering this sports bra especially considering the vague sizing guideline. Boy, am I glad I ordered two! This is a wonderfully comfortable and usable nursing sports bra. I am petite yet fully endowed, so I ordered size large. They fit well and the nursing openings are actually big enough so that I don't have to squeeze my breast through a tiny slit way off to the side which is usually the case with nursing openings in my tank tops, etc. The clips are very secure, too. Overall, a great bra. I just wish I didn't have to consult a different website to check for sizing.