How effective are contraceptive sponges?

Best if no delivery: The sponge is more effective for women who have never given birth. If women who have never given birth always use the sponge as directed, 9 out of 100 will become pregnant each year. If women who have never given birth don't always use the sponge as directed, 12 out of 100 will become pregnant each year. 20/100 and 24/100 if previously given birth vaginally.