How likely is it to become pregnant on birth control pills?

Small: If you took your pills every day without missing even once, then the chance is under one percent in a year.

Dual methods: Can be as high as about 5%. Most people recommend that you use dual methods for birth control, e.g. The pill and condoms. See: http://goo.Gl/ql4bs gives 5%; http://goo.Gl/9lwyc gives 8%; http://goo.Gl/hefsv gives 2%; http://goo.Gl/9hjyp gives 5%. Note that the failure rate given on these sites list Depo Provera (medroxyprogesterone) (hormone injection instead of pills) as having a significantly lower failure rate.

Very unlikely: Birth control pill companies like to quote statistics that the pills are 99% effective. Maybe under the most ideal of circumstances, but most experts agree that in practical real life use, the pill about 97-98% effective. That means an annual risk of about 2-3% chance of pregnancy. Add condoms, and you approach 0% risk of pregnancy.