We are commonly asked about the effectiveness of Birth Control. And specifically, does oral contraception work? Here are a few answers to your questions. Studies have found that, used as recommended, oral contraception is 91 percent to 99 percent effective in preventing pregnancy. However, that still means there is a very small risk of pregnancy. In most instances, however, women who become pregnant while taking oral contraceptives either miss one or more doses; take a dose at a time different from their normal time (i.e., in the evening instead of the morning); take medications (such as antibiotics) that interfere with the effectiveness of birth control in your body; or have unprotected intercourse too soon after starting their oral contraceptive. If you begin taking birth control pills within six days of the first day of your period, it's effective immediately. If you start at any other time, however, you need to take it for a full month before it's effective, and you must use another form of birth control until you've completed one full cycle of pills. While no method of contraception other than sterilization is 100 percent effective against pregnancy, the more precautions you take, the less likely you are to get pregnant. Plus, if you are not in a long-term, monogamous relationship, your partner should be using a condom to protect you both against sexually transmitted diseases. Talk to your physician for more information about contraception options and questions.