An increasing number of American teenage girls say they use the rhythm method for birth control - which could explain why the teenage pregnancy rate is no longer falling.
Associated Press reports that a government survey in the USA found that 17 in 2002.
The rhythm method is an unreliable form of avoiding pregnancy, where women time when they have sex to avoid their fertile days.
Joyce Abma, the report's lead author, and a social scientist at the CDC's National Centre for Health Statistics, told AP that the rhythm method doesn't work about 25 of teenagers who had never been married had had sex at least once.
Of those teenagers, 98 of teenage boys and 70 and 65% in 2002.
While the survey was taking place there were a number of high-profile unmarried mothers including Bristol Palin, the daughter of the vice-presidential candidate Sarah Palin, and Jamie Lynn Spears, Britney's younger sister.