Natalie, I would keep up with the exercise if I were you. I have a friend who is a pretty high level athlete (she's actually in the Guinness Book of WW for stroller running now) and she did marathons her entire first pregnancy - she even phoned her doctor to see if she could run another one after her water broke! She's pregnant again and continuing to keep at it. I was told that as long as your heart rate stays under 140 you're not doing anything wrong. I think the whole hb thing is debated all over the place, but google google google
Find something you're comfortable with, get yourself a heart rate monitor, and just don't do any heavy lifting or weird diets. ;-) Or push ups. I really messed my wrists up doing pushups at 35+ weeks, lol....