I suggest reading "Exercising Through Your Pregnancy" by Dr. James Clapp. It highlights (through research) the tremendous benefits of exercising while pregnant (pre-pregnancy, antepartum and postpartum).

As an aside, I ran until I was 36 weeks pregnant, including several 5 and 10ks and a marathon relay. I used the support belt later on and it does work well while running. I had a very easy pregnancy, labor and delivery and a healthy baby girl who is doing great. I highly recommend exercising. It really kept my sanity and stress in check during pregnancy, and it sure helps with those postpartum hormones afterwards!

I'm still doing cardio 5x a week and lifting weights 2x a week. Midwife says that is just fine, except she'd prefer I modify so that I don't put weights over my head (mostly so I don't topple over from balance issues as my belly gets bigger).

Baby boy is doing well, growing perfectly, and gets lulled to sleep every time I'm on the treadmill (midwife says it's all the bouncing!)

Exercise is really, really good in pregnancy. I'm also happy to give you all the research I did.