Quoting CRSx2Mommy:" I think it might depend on her fitness level pre pregnancy. If she was already in super good shape and ... [snip!] ... good shape and worked out all the time then continuing to do so shouldn't hurt her although she should tone it down a bit. "
I've heard people say you can continue doing in pregnancy what you did pregnancy but my own doctors told me to tone down my activity. I was doing yoga, pilates, running, and various Jillian Michaels DVDs prior to getting pregnant and switched over to prenatal yoga and walking. Not to say my doctors are more right than others, but it's what made me think doing so much could be dangerous.