You may have heard of Body Balance. Body Balance is a mixture of Yoga, Pilates and Tai Chi, it has been around for quite a few years now and most larger gyms will have a class. I really love it as it is low-impact, strengthens your core and other stability muscles, and is the perfect combination of balancing, strengthening and stretching.
As mothers, after pregnancy and carrying young children, we often end up with back pains, core weakness and other niggling pains. If you suffer from any of these and you haven't tried it, I highly recommend giving it a go. It's not a strenuous cardio workout, but it does wonders for your back, flexibility and general feelings of wellbeing. There is usually a 10 minute relaxation at the end of the class too (which can be heavenly for us busy mamas).
You can also participate in BB while pregnant, just make sure you inform your instructor beforehand so they can modify certain exercises for you.
Check your local gym timetable and see if they offer it. Have a go and let me know what you think over at the facebook group.