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 someone has been pregnant before and what size cap they use.

In other words:

14 out of 100 people who have never been pregnant, have not had a vaginal birth, and use small or medium sized cap, will get pregnant when they don’t want to during the first year they use the cap.

29 out of 100 people who have had a vaginal birth or use a large size cap, will get pregnant when they don’t want to during the first year they use the cap.