I'm a personal trainer and Les Mills fitness instructor in body attack, body balance, body combat, body jam, body pump and RPM. I know I can't reference other links but you can definately do keyword searches if you want to know more about the classes on the internet.

My GP told me not to get too hot in especially the 1st trimester, so I ended up no longer teaching attack and combat, however I'm going to stick with instructing body pump - with light weights, body balance and RPM (not as full on, and I won't be instructing this one as I'd get too hot). And yes, don't go too hard on the ab's - maybe just get a fitball and sit on the ball - lift one leg off of the ground, and swap legs. It gently works the ab's building core strength.

GP's will also say to stay away from bike riding (stationary bike is ok though) horse riding, skiing, anything where there is a chance you may fall off and take a hard hit as you land (no matter how accomplished you are ) - for bubba's sake!

Keep your fluid up while exercising, and it's best to start exercising before you fall PG...

I hope this helps a bit.