I'm quasi-medical; while not technically qualified yet to give advice to others, I'll offer up a few words.

This article seems to contain pretty sound advice:

http://www.velonews.com/train/articles/3767.0.html

And please remember that as long as you don't drink/smoke/do drugs, etc., most problems with pregnancy and childbirth aren't due to anything that you do directly to your baby or yourself - they're due to subtle genetic defects that you can't do anything about. So enjoy your pregnancy and keep riding at a reasonable pace - best of luck!