You don't need to pay the "pink tax" and buy a ball called a Birth Ball. Any yoga/pilates/exercise ball will do.
I like this one because it comes with a guide of ways to use your ball, so you don't get stuck in a rut. Remember, it isn't just for your bum. You can kneel and lean over it, stand and lean over it (on a surface), put it between you and the wall and use it to glide down into a squat. Get creative!
You do want one that is the right size for you. They generally come in 55cm/65cm/75cm. If you are under 5'2", get the 55cm. If you are over 5'7", get the 75cm. Anywhere in between and the 65cm is right for you. And inflate it all the way. It shouldn't bulge out much when you sit on it. You want to sit on a ball not a beanbag chair.
