How effective is the cervical cap at preventing pregnancy?

This one’s a little confusing. We only know about the effectiveness of the cervical cap based on typical use (not perfect use) and it’s between 71% and 86%, depending on whether a woman has been pregnant before and what size of a cap she uses.

In other words:

14 out of 100 women who have never been pregnant, have not had a vaginal birth, and use small or medium sized cap, but do not use the cervical cap exactly as directed will experience an accidental pregnancy during the first year they use this method.

29 out of 100 women who have had a vaginal birth or use a large size cap, but do not use the cervical cap exactly as directed, will experience an accidental pregnancy during the first year they use this method.