Not necessarily. Depends on what you mean by "think about it", I suppose. Like, if I concentrate on delivering the words carefully, do I "think about stutter"? Or if I think about saying something, knowing I'll probably stutter, and then go on to say it without difficulty - that's not a case of it getting worse. Perhaps what you mean is something like "will being nervous about stuttering make me stutter more"? This might be more or less likely, but I don't think it's like that all the time, no.