I've heard that the pregnant female body is far more aero than the regular shape, due to the high-efficiency tear-drop shape. Windtunnel testing, done at great expense, has shown getting pregnant to be one of the highest performance upgrades you can make. Just make sure you are wearing proper lycra at all times, to eliminate any source of turbulence around your now perfectly aerodynamic body.

Seriously though, the 'old model' for pregnancy amounted to, "Do nothing at all, sit or lay in your bed all day reading romance novels until you have your baby." That isn't the case anymore, but there are definately some holdovers. If you are in good physical shape now, I don't think your doctor would have a problem with you continuing to cycle until fairly far along in the pregnancy. You may appreciate a more upright riding position however. It is my opinion (and I think it is shared by many doctors), that as long as you are careful about it, maintaining physical activity for as long as possible through the pregnancy can only be good for yourself and the baby. A healthy mother will make a healthy baby, and exercise is a part of that picture.

Good luck!

Peace,
Sam