I'm preggo with twins and my doc said walking, yoga and swimming were the best but no bouncing or high impact exercises.
I walk almost everyday. My OBGYN actually encouraged exercise, however be safe and listen to your body. Since you had a routine prior to pregnancy your body is use to it somewhat.
I think as long as the exercise is not strenuous..and you're not over doing it you will be just fine. The best ive heard is walking, I used an elliptical routinely before I got pregnant, I dont anymore because ive just not had the energy ( I also have 2 boys that are 4 and 2) and I work as a server in a restaurant 6 days a week. So I feel like with working and taking care of my boys I get enough in. Just no lifting!