I was riding just locally up until about 30w. I am just too squished in the lungs to be able to do it anymore and I have kneed myself in the belly once to often to make it comfortable. I found that I had to adjust my style - knees out more and sitting more upright as my belly grew.

I am not a mountain biker (more of a roadie) so I can't help you on that but as your belly grows you may notice that your centre of gravity changes making it more difficult to balance on rough terrain. I also found that I didn't feel as confident riding in heavy traffic (maternal instinct maybe) although my route to work is all through inner city Melb and across the CBD so it is probably understandable - especially sice a pregnant cyclist was hit and killed by a bus on Swanston St - so I stoped riding to work early on and stuck to my local area.