It is important to buy a birth ball the correct size. For an average woman 5'5" this will be a 65cm ball. If you are much shorter you'll want the smaller 55 cm size or much taller you'd want the larger size. Check to make sure it's the right fit for you by sitting directly on top, feet flat on the floor. Your legs should be at a 90 degree angle. If your knees are too high it may cause strain on your hips.
Re: One more ? about exercise ball
This is what I got from a google search...
Selecting Your Birth Ball
It is important to buy a birth ball the correct size. For an average woman 5'5" this will be a 65cm ball. If you are much shorter you'll want the smaller 55 cm size or much taller you'd want the larger size. Check to make sure it's the right fit for you by sitting directly on top, feet flat on the floor. Your legs should be at a 90 degree angle. If your knees are too high it may cause strain on your hips.
https://www.associatedcontent.com/article/74611/using_a_birth_ball_during_pregnancy.html