Hi Kate.Colo! So I worked browns canyon (class III) every day all through my first trimester, and my best friend floated the Truckee (class I) as much as she could during her 3rd trimester. I met Telegirl rigging her raft at stone bridge with a big belly (not sure exactly what month at) to run a class II section of the Ark a few years back. So I am going to say YES! Give that baby his/her first few River miles in utero!
All of the aforementioned bellies are now accomplished little River Kids! (Mine turns 5 tomorrow & has almost 400 [out of utero] River miles under her belt!) I'd also say that you're probably right about hiking being worse than running the rapids. The main concern with boating pregnant is the possibility of taking an oar to the belly, but if your not the one rowing & you trust him at the oars, then I'd say stay on the boat & run it!
Also, welcome to the wonderful world of Mommyhood! Congratulations! And yay for another new boater being born!
If you do the facebook thing, I'd like to invite you to join a fantastic group called River Mammas that is a great source for all things River & kid related, & just a fantastic community of boating parents! http://www.facebook.com/groups/250132875077406/?fref=ts
Anyways, again, CONGRATULATIONS! & YES DO IT!