Great links on buying and using a birth (exercise) ball during pregnancy, labor and postpartum:

http://annabel-yogabirth.com/articles/birthing-balls/

http://pregnancy.about.com/od/productsforlabor/ss/A-Birth-Ball-Offers-Uses-For-Pregnancy-And-Newborn-Days.htm

Videos:

https://www.youtube.com/watch?v=bD5yBrqePHg

https://www.youtube.com/watch?v=F20Ny-NPunw