There is no real limit. It is up to you to supervise the bub during exercising. You will be in a gym with equipment, so obviously we don't want a child running around grabbing bits and pieces.
Once you get the go ahead from the doc, you can come ahead. If you have had a c-section I advice a personal training session before a class.
No matter when you join, you will be brought through basic movements according to your abilities. These will be aimed at increasing cardio, full body and core strength. That means bodyweight work to start with.