There have been a few studies to show that excessive running and very vigorous exercise in the first trimester can increase risk of m/c, but having said that lots of people continue to do so without any worries. I would keep it moderate to easy in the first trimester. You want to make sure that your body temperature doesn't raise too high while exercising (same as in a bath - keep it warm not hot) but moderate exercise is definately great to do. If you are keen to keep doing your group classes, let your instructors know and they will tell which exercises you can and can't do. I did a normal aqua class and I just didn't do anything "sit up" like etc.
Good luck with it.