Under review technically means they haven't made a decision about whether to invite you for an interview yet. However, I think a number of schools use this status in place of a "hold" or when they've already decided to reject people but haven't started sending out rejections yet. I mean, they're not going to be like, "Oh yeah, you got the boot, but you won't get a letter for a few months yet. You didn't hear it from me." In other cases, you might very well still be under review, especially if you didn't turn in your applications super early, or if you're sort of borderline and they're waiting to see what else comes around (not necessarily the same as a hold).
I would try to relax and just keep chugging along. If you're still in school, send updated transcripts when you get your grades for this semester. If you did well, it might bring your application back to the top of the pile and give you another chance.
