The problem with belly belts is that they don't come in black.

As far as how fast you pop out - it's to do with lots of things:

First or subsequent pregnancies (you get bigger second plus times around - already stretched)
How long your body is - you can be short with a long body and so hardly get big at all, or tall with a short body (especially if you're short waisted) and so will pop out quickly as there is nowhere else for the baby to go.

I'm only 4 weeks and already I can feel that my waist is bigger and the waistbands on all my clothes are starting to hurt me.