Women should not get their first cervical cancer screening before age 21, the leading US group of women's health care professionals said Friday, also recommending less frequent subsequent tests.

Pushing back the age of the first screening would help avoid giving teen girls unnecessary treatment, which can have "economic, emotional and future childbearing implications," according to the guidelines issued by the American College of Obstetricians and Gynecologists (ACOG).

ACOG previously recommended that cervical screening begin three years after first sexual intercourse or by age 21, whichever occurred first.

But the guidelines were revised because "screening for cervical cancer in adolescents only serves to increase their anxiety and has led to overuse of follow-up procedures for something that usually resolves on its own," said Alan Waxman, who led the ACOG team that drew up the recommendations.

The new guidelines also recommend that most women under 30 be screened every two years instead of annually, and that women older than 30 who have had three consecutive negative cervical cytology test results be screened once every three years instead of yearly.

Certain women, including those carrying the HIV virus, with suppressed immune systems or who have been treated for cervical cancer, may need to be screened more often, the guidelines said.

ACOG made its revisions after a data review showed that "screening at less frequent intervals prevents cervical cancer just as well, has decreased costs and avoids unnecessary interventions that could be harmful," Waxman said.

Cervical cancer rates have fallen by more than half in the past 30 years in the United States, thanks to the widespread use of the Pap smear test, ACOG said.

In 1975, nearly 15 women per 100,000 in the United States had the slow-growing cancer, which is caused by certain strains of the human papillomavirus (HPV).

Thirty-one years later, in 2006, the rate was 6.5 women per 100,000.

The new recommendations are to be published in the December issue of Obstetrics and Gynecology.