Ab work is best avoided, as are some back exercises (because some of the muscles on the back connect to the front). I would suggest talking to a GP and also a trainer at your gym who is qualified to work with pregnant women. all other exercise that your body is used to and already comfortable with should be fine, so long as you are conscious not to overheat (and if you do, not for more then a 15 minute period) as this could harm the fetus.

hope this helps, I had to research this because I was seriously into hockey. Unfortunately I had to give it up anyway, due to insurance reasons.