Number one rule of being pregnant is to listen to your body.
I rode a mountain bike on bike paths unitl my protruding tummy got too big. You will instinctively know when to quit as your balance will be off and your knees will hit your belly. I'd recommend a mountain or hybrid stlye bike over a road bike as it's most likely more comfortable. It's be common sense to stay out of traffic and other potentially risky situations but only you can decide what those are, for you.
Swimming is probably the best exercise for pregnancy as it's so low stress on the joints. And, when you are the size of a whale it feels sooo good to float in the water.
Irulan