I am 5'6 tall, 10 weeks pregnant. I ride a lot and fast. I feel much better on the bike. As soon as I get off the bike I am unhappy. My doctor tells me just to listen to my body. I ride several times a week at least 20-22 miles and longer on the weekends. Just do whatever feels right. If you start feeling uncomfortable, slow down. The rule is definitely eat something before th ride. I didn't use to do that especialy early in the morning. But now I have too.
Good luck and congratulations!