According to WebMD, birth control pills are normally effective after the first month a woman takes them. It is recommended to use an additional method of birth control during the first month.
Birth control pills contain hormones that inhibit a woman's normal cycle and prevent her body from becoming pregnant. Taken correctly, birth control pills are up to 99.9 percent effective. A doctor advises a woman on when the pills should be started.
Typically, a woman begins birth control pills the first Sunday after a menstrual period starts, even if she is still bleeding. After 1 month of taking the pills, backup forms of birth control are not needed. Women can rely solely on the pill for pregnancy protection.