TORONTO -- The Society of Obstetricians and Gynecologists of Canada and two related medical organizations are taking issue with a national task force's recommendations that women wait until age 25 to start cervical cancer screening.

In a newly released position paper, the SOGC, the Society of Gynecologic Oncology and the Society of Canadian Colposcopists say age 25 is too late to begin Pap testing because precancerous and cancerous lesions may have developed earlier in some women.

The groups say young women should have their first Pap smear at age 21 and have them done every two to three years.

The Canadian Task Force on Preventive Health Care, which issued national guidelines in early January, advises that women wait three years between screenings, which should continue to age 69. For years, women were told to get annual Pap smears, though that interval has been stretched in recent years in a number of countries.

Dr. Jennifer Blake, CEO of SOGC, said the task force was correct in saying that waiting until 25 would have little effect on the number of women under 25 who die from cervical cancer, which usually is a relatively slow-growing cancer.

"But we do think that you can find -- and we do find -- advanced lesions in young girls or even if we find early lesions in young girls, just by doing very minor things, removing those abnormal cells even in the process of a biopsy, you impact the natural history of the disease," Blake said Thursday from Ottawa.

"And what can happen then is if you do end up needing to treat, treatment is less destructive," she said, explaining that having to remove a larger part of an otherwise healthy cervix can affect a woman's fertility.

That was a major concern of the task force, which suggested that Pap smears that turn up abnormal tissue can lead to overly aggressive treatment in young women that can affect future fertility. Treatment can result in an inability to carry a pregnancy to term.

"That doesn't mean you don't start screening," said Blake. "That means you don't over treat."

"So we now say that if you are seeing problems in younger women, you can safely watch them with conservative treatment, just doing a biopsy and watching."

But Dr. James Dickinson, chair of the task force, said age 25 was recommended because evidence shows that cervical cancer is rare in young women, even up to age 30, and "vigorous screening has not reduced cancer much at these young ages."

Dickinson, a professor of family medicine at the University of Calgary, said 10 per cent of Pap smears are positive in women up to age 30 and require at least one cervical biopsy and possible treatment that can lead for some to miscarriage or premature labour.

"Those women, those young women, have a risk that this can cause trouble because the cervix has a function -- it holds babies in. And part of that function may be lost," Dickinson said from Gold Coast, Australia, where he is on sabbatical at Bond University's Centre for Research in Evidence-Based Practice..

Women can also suffer psychological harm, worrying about whether they have cancer as they wait for biopsy results, he said.

Dr. Betsy Brydon, president of the Society of Canadian Colposcopists -- the doctors who perform cervical biopsies -- said new guidelines were issued in December aimed at reducing potential harm from diagnostic and treatment procedures, and discouraging aggressive treatment unless clearly warranted.

"So we try not to treat when we don't need to treat, and when we do treat, we try to limit the amount of skin that we remove (from the) cervix," she said from Regina.

Dickinson said the task force, which bases its recommendations on international studies and epidemiological evidence, attempts to balance potential benefits against potential harms.

Much of the data comes from other countries, and the three medical organizations called that evidence "weak."

Blake said there is no data on Canada's female population that would show it is safe to wait until age 25 to start screening or for waiting three years between tests -- an interval she suggested women would find difficult to remember.

"We should not be interpreting evidence from other countries and applying it to our own young women unless we really have very good certainty that it's a safe thing to do," she said.

"We're saying let's err on the side of being conservative ... because (cervical cancer) remains the second most common cancer in young women (aged 20 to 44) and it still has a significant impact on mortality and loss of fertility and sexual function."

Dickinson said citing the 20-44 figure is misleading because cervical cancer is still relatively rare: without screening, a woman's lifetime risk of developing cervical cancer is 1.5 per cent.

The Pap smear is one of the most effective screening procedures that doctors have, he said, and it's reduced the incidence of cervical cancer by 80 per cent.

"That's really great. But we've got to balance that against the fact that for 98 per cent of women who are never going to get it, having Pap smears is just a thorough nuisance and causes some harm."

The Canadian Cancer Society estimated that in 2012, 1,350 Canadian women were diagnosed with cervical cancer and 390 died from the disease.

Research shows most of the benefit of cervical cancer screening occurs for women in their 40s and older, the point at which most cervical cancer cases are diagnosed.

However, abnormal test results are not uncommon in younger women because the Pap smears pick up lesions caused by infections with the human papillomaviruses (HPV), which can cause cervical cancer.

Many of those lesions would heal on their own and would not go on to become cancer. But once they are found, women often need additional testing and treatments, some of which can make it hard for a woman to later carry a pregnancy to term.

As vaccines to prevent HPV infection become more widely used -- they are now recommended for females age nine to 49 and males nine to 26 -- screening guidelines will likely change.

Meanwhile, screening with the Pap smear and expanding HPV testing across Canada remain critical for preventing cases of cervical cancer and cervical cancer deaths, she said.

Blake said the emphasis needs to be put on getting females to have regular Pap smears: more than half of Canadian women diagnosed with cervical cancer had never been screened, while the rest of those diagnosed could have had a Pap test, but it either was not done recently or there had been a false negative.

As well, a recent Toronto study showed one-quarter of women who had an abnormal Pap smear did not have follow-up tests or treatment.

"The biggest problem with cervical cancer in Canada isn't the frequency of screening, it's not the age of initiation of screening, it's not whether or not we do HPV co-testing," said Brydon. "The problem is that women don't go for testing."

That's one thing Dickinson agrees with -- a push is needed to identify groups of women who are not getting tested, possibly because of living in rural or isolated communities with poor access to services or for socioeconomic reasons.

Blake conceded that young women may be confused by the conflicting advice being given by two groups of credible medical professionals, making them unsure when they should start being tested or how often.

"I think the message is that there's a difference of opinion and you may want to talk to your doctor and make a decision yourselves," she said.

Brydon suggested women go online to check screening guidelines being used by their individual province or territory or ask their doctors.