I am 21 weeks and I have a bump, but its not huge, I have always pushed and worked out hard, butr am trying to tone in down a bit. I"ve found my self nervouse while working out latley. I have been doing crunches and sit-ups (as usual). But a friend told me and I read that I shouldn't be lying on my back. I read that this is b/c our uterus block O2 flow to our lil babies. I have a trainer, but she is a friend and has always trained athletes, never prenatal workouts. Thoughts on intensity during working out, and being on your back.
Re: Question about second trimester workouts
My midwives have said that you should stop lying on your back when you feel uncomfortable. When the weight of your uterus affects your blood supply, you will know it. You might get pins and needles, or a tingling, or just a weird feeling in your legs.
I stopped doing crunches @15 weeks, but still do other ab workouts - planks, side planks, oblique work, and such.
http://oi62.tinypic.com/2w73hq9.jpg
Without a doubt you do not want to be doing supine exercises while pregnant and your trainer should know that.
As for intensity, I would use your body as a guide. In my 2nd trimester I was doing from 1 hour 15min, to 1 hour and 45 minutes of cardio and weights. I never pushed myself to the limit and I felt comfortable 100% of the time, otherwise I wouldn't have done it. I didn't lift as much weight as before and tweaked each exercise according to how I felt (lighter weights or lower reps).
Oh, and I stopped doing crunches as well as I found them uncomfortable. You can find alternative core exercises like planks..