What Andrea said.
I found that during the middle part of the first trimester I couldn't do much more than a light spinning workout. I would force myself to get on the bike and just spin and I found that my energy increased enough to finish the workout. But that exhaustion really only lasted until the 8th week or so and then I had other symptoms to deal with. Keep in mind that as long as you stay moderately active you can pick it back up in the second trimester. I'm at 15 weeks and doing CF 3-4 times per week and then spinning the other two days. I feel great and have loads of energy, so your workouts can increase when you hit that stage.