Taking birth control for less than a week has little effect on the body and menstrual cycle. According to Military Obstetrics & Gynecology, birth control pills do not achieve their full effectiveness until after the first month of use. Many physicians and pill manufacturers suggest that birth control pills become effective after seven days of continuous use.
According to Military Obstetrics & Gynecology, a backup form of birth control, such as a condom, is advised if a woman skips two or more birth control pills. This is due to the fact that birth control pills may lose their effectiveness when two or more pills are skipped. The site also says that some doctors recommend using an alternate form of birth control during the first month of starting birth control pills; this is because they do not reach their 99.9 percent effective rate until after a month of continuous use.
Dr. Amos Grunebaum, medical director if the WebMD Fertility Center, states that the hormones from birth control pills leave the body within a few days, whether the user has been on birth control for one week or 10 years. After stopping birth control, ovulation is no longer prevented, and the body returns to its natural cycle; however, it takes time for the hormones in the birth control pills to affect ovulation. Ovulation may begin within a matter of weeks or months, depending on the person.