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!