The effectiveness of the sponge depends on whether you've had a baby or not. When used perfectly, it's about 91% effective for those who have never had a baby and about 80% effective for those who have. With typical use, effectiveness goes down to around 88% for those who have not had a child and 76% for those who have. **In other words, among women who have never given birth:**   Of those women who use the sponge exactly as directed, about 9 in 100 will experience an accidental pregnancy during the first year of using this method.    Of those women who do not use the sponge exactly as directed, about 12 in 100 will experience an accidental pregnancy during the first year of using this method.    **And among those who have given birth:**    Of those women who use the sponge exactly as directed, about 20 in 100 will experience an accidental pregnancy during the first year of using this method.  Of those women who do not use the sponge exactly as directed, about 24 in 100 will experience an accidental pregnancy during the first year of using this method.