IF you take your pill perfectly, without fali, at near the same time every single day, and have no factors (other medications, herbal medicines, changes in diet or homones) that can make your pills less effective, on average, about 2% or a 2 in 100 chance of pregnancy.
I should add though that even when parttners test utterly clear of STDs and STIS, that does not mean you cannot get those infections, or pass other bacterial infections.
At Scarleteen STIs aside, we advise that if you do NOT wish to become pregnant, you use condoms with the pill.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson