Seventeen percent of sexually active teenage girls said they used the rhythm method as a means of birth control in 2008, up from just 11% in 2002, according to the latest report from the CDC. For most of these girls “rhythm method” means guessing the least risky day to have unprotected sex. You and I both know that one in five teenage girls isn’t taking her temperature every day and charting the consistency of her cervical mucus on the calendar.