Birth control pills are used for several reasons.. first of all the most obvious reason is to help a woman to not become prego.. but it is not 100 %.. so always use another alternative with the pill....
If you're referring to the birth control pill, you should use a back up method of birth control, like condoms or abstinence from vaginal sex, for the first seven days of the first pack.
You take them exactly as prescribed by the doctor who wrote the prescription for this medication. If the directions are not followed to-the-letter, the possibilities of becoming pregnant increase...