Im definately no gym junkie but was doing 2 pump classes a week and a fitball class for about 8 months prior to falling pregnant. I still continued, but did the following

......gradually dropped my weights down so I didn't put too much stress on my body.
......stopped abdominal exercises fairly early
......used a fitball on wall for squat track when I started getting bigger.
......didn't lie on back
......stopped at 30 weeks when my back couldnt take it any more.

GL