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% of teenage girls with sexual experience said they had used the rhythm method, up from 11% 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 the time.

The researchers carried out face-to-face interviews with nearly 2,800 teenagers aged from 15 to 19 at their homes between 2006 and 2008.

The survey discovered that about 42% of teenagers who had never been married had had sex at least once.

Of those teenagers, 98% said they had used birth control at least once, usually condoms.

AP reports that the teenage birth rate went down steadily between 1991 and 2005, but rose again from 2005 to 2007, with a small drop in 2008.

Bill Albert, a spokesman for the National Campaign to Prevent Teen Pregnancy, told AP: "We've known the decline in childbearing stalled out. This report kind of fills in the why."

The study also found that nearly 64% of teenage boys and 70% of teenage girls thought it was okay for an unmarried woman to have a child, up from 50% 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.

Source: ParentDishUS