Circuit Training/Core/Flexibility

I bought the "Super Fit Mama" book almost a month ago and I LOVE it!!!! (Tracey, thank you so much!) I'm 19 weeks pregnant. I have been doing the circuit training/core/flexibility Monday, Wednesday, and Friday mornings and have been doing 30 minute cardio Tuesdays, Thursdays, Saturdays. I've been feeling great...anyway, my question is, is there anything I can add to the circuit training and core strengthening sections to intensify my workout? I was pretty active before I got pregnant, and I feel like I need help making the workouts a little more challenging...not much, but a little more. Any suggestions?