I use one all the time. It’s really comfortable and conforms to your body rather than sitting on a stiff chair. At my childbirth class they suggested sitting on one during pregnancy, and bringing it to the hospital for labor.
Just make sure it’s a decent quality and burst-proof, so if you roll over something sharp it won’t explode or anything.

check with your doctor, but I have heard that doing ab exercises are not good while pregnant.

JP,

Check with your doctor but I think you have a great idea. You will need your core strength during labor, and what a great way to maintain it while you’re carrying.

yes it will help tighten and tone your pelvic floor muscles. Great for pushing out baby. but if you are worried just ask your doc that is what they are there for.

Im sure there is nothing wrong with it as long as you arent worried about losing your balance when sitting on it for long periods of time. I would be worried about falling off.