I rode until my belly got in the way - at about 25 weeks. I think if you are a good cyclist and are riding cautiously you should be OK. I would stick to paved trails as much as you can... Oh, and first trimester I did everything I normally did including downhill skiing with my doctor's blessing. The doctor said at that stage the only way I'd hurt the baby was if I put myself into the hospital with severe injuries. So I definitely wouldn't worry too much right now.

Congrats on your pregnancy!!