20-week scan

We've got the 20-week scan next Tuesday, and for the last couple of
days I've had niggling worries about it. Don't really know what about
it. Kind of the "What if..." thing, I think. I woke up last night and
was laid awake for a bit thinking about it. I really want it, because
I want to see Bubble, but I guess I'm scared that there's something
that I'm not going to like. I've got no groundings for this at all,
and maybe it's just me.
I mentioned it at the yoga class today, and it was pointed out that I
don't have to have it if I don't want it. Should I have it to set any
worries to rest? Should I not have it and just trust that nature will
make it's own way and everything will sort itself out?
I know Dave is looking forward to it, but I just don't know. It's all
quite scary - more so than the 12-week one, for some reason - and I
don't know what to think.