More teen girls are using the rhythm method to prevent pregnancy, according to a new survey by the Centers for Disease Control and Prevention; about 17 percent of teen girls who've had sex use the method, up from 11 percent in 2002. The development is troubling because the method—abstaining from sex when women are fertile—has a 25 percent failure rate. The CDC also found that 42 percent of teens who'd never married had had sex at least once, and 98 percent of those teens had used birth control at least once, usually condoms. The increased use of the rhythm method could explain why the rate of teen pregnancy, which declined from 1991 to 2005, began rising through 2007 before dropping slightly in 2008. About 10 percent of all babies are born to teenagers. Changing attitudes may be playing a role, too. Nearly 64 percent of teen boys (and 70 percent of teen girls) believe it's OK for an unmarried woman to have a baby, up from eight years ago, when only 50 percent of boys said it was acceptable.