They don't really have the freedom to do that anymore. At least not in the U.S.
The only way they'll bump people up, is if they paid full fare for their ticket (there is a code in your boarding pass that tells them if you did), if they have been bumped from another flight and the only available flight ONLY has first or b class seats available.
Also, flight manifests are much more accurately kept now (post 9-11) and they are not supposed to move people around.
You could possibly ask to be put in an exit row? Or if you fly JetBlue, they'll sometimes upgrade you to an "even more space" seat.
I don't mean to sound like a jerk, but usually accommodations like that would be made for pregnant women because they are large and uncomfortable. If you're not really showing (your ticker says you're 11wks), then you take up about as much space as a regular person and they'd likely not have much motive to move you (except maybe closer to the bathroom!)