Most women in their 20s can have a Pap smear every two years instead of annually, say new U.S. guidelines that conclude that is enough to catch slow-growing cervical cancer.

The change by the American College of Obstetricians and Gynecologists comes amid a completely separate debate over when regular mammograms to detect breast cancer should begin.

The timing of the Pap guidelines is coincidence, said the ACOG, which began reviewing its recommendations in late 2007 and published the update Friday in the journal Obstetrics & Gynecology.

The guidelines also say:

• Routine Paps should start at age 21. Previously, ACOG had urged a first Pap either within three years of first sexual intercourse or at age 21.
• Women 30 and older should wait three years between Paps once they've had three consecutive clear tests. Other national guidelines have long recommended the three-year interval; ACOG had previously backed a two- to three-year wait.
• Women with HIV, other immune-weakening conditions or previous cervical abnormalities may need more frequent screening.

Paps can spot pre-cancerous changes in the cervix in time to prevent invasive cancer and widespread use has halved cervical cancer rates in the U.S. in recent decades.

To minimize the risk of cervical cancer, Health Canada's website recommends:

• Have a Pap test at age 18 as part of your routine health examination, or as soon as you become sexually active. A second test should be taken after one year, especially if you begin screening after age 20.
• If your first two tests show no abnormality, you should be re-screened every three years to age 69. However, you do not need to be re-screened if you have never had sexual intercourse or if you have had a hysterectomy and your previous tests were normal.
• If you are over the age of 69, and have had at least two clear Pap tests, no cervical abnormalities for nine years and no history of cancer, you do not need regular screening.
• If an abnormality is detected during a Pap test, you should be retested every six months for two years.
• Limit your number of sexual partners and be aware of your partners' sexual history.
• Limit your number of unprotected sexual encounters. Using condoms also lowers your risk.

Cervical cancer is caused by certain strains of the extremely common sexually transmitted virus called HPV, for human papillomavirus.

There is a new HPV vaccine that should cut cervical cancer in the future. ACOG's guidelines say for now vaccinated women should follow the same Pap guidelines as the unvaccinated.

Nature of infections

But the updated guidelines reflect better understanding of HPV, such as how infection is high among sexually active teens and young adults.

Women's bodies very often fight off an HPV infection on their own without lasting harm, although it can take a year or two. The younger the woman, the more likely that HPV is going to be transient.

Moreover, ACOG cited studies showing no increased risk of cancer developing in women in their 20s if they extended Pap screening from every year to every two years.

As for adolescents, ACOG said cervical cancer in teens is rare — one or two cases per million 15- to 19-year-olds — while HPV-caused cervical abnormalities usually go away on their own, and unnecessary treatment increases the girls' risk of premature labour years later.

According to the Canadian Cancer Society, incidence rates of cervical cancer have declined two per cent per year from 1995 to 2004. Still, nearly 1,400 Canadian women were diagnosed with cervical cancer in 2006.