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OTTAWA - In a finding that is expected to rewrite medical textbooks, Canadian researchers have discovered that, for many women hoping to avoid pregnancy, there is no "safe" time to have sex.

For 50 years, doctors have believed that about a dozen follicles, or egg sacs, grow at one time during a woman's menstrual cycle. From this group, only one follicle actually bursts and releases an egg, while the others shrivel and die.

But, in a finding that left even the researchers "flabbergasted," University of Saskatchewan scientists have found this pattern of follicular development actually occurs two to three separate times during a woman's menstrual cycle.

What's more, the scientists found, 40 per cent of women have the biological potential to ovulate more than once during a cycle.

"The old idea of one time per cycle is wrong," says senior author Dr. Roger Pierson, director of the reproductive biology research unit at the University of Saskatchewan in Saskatoon.

The results help explain for the first time why some women get pregnant while on birth control pills, and why the window for safe sex may not exist at all for many women -- because there may always be an egg sac waiting to release an egg.

"We all know people trying to use natural family planning, and we have a word for those people. We call them parents," says Pierson.

The discovery could lead to more effective contraceptives and could boost success rates for women undergoing expensive and invasive high-tech infertility treatments.

The study, funded by the Canadian Institutes of Health Research, is published this week in the journal Fertility and Sterility.

For decades, the medical dogma held that a woman's ovaries behaved "one very certain, very specific, very predictable way," Pierson says. Normally, women have a 28-day menstrual cycle, and the belief was that most women ovulate once, around day 14. The cycle begins on the first day of bleeding. By about day five, the theory held, about 15 to 20 egg sacs start to mature and then, by day 14, the most mature follicle ruptures, releasing an egg.

But, those assumptions were largely based on blood samples and menstrual diaries. "We actually looked at the ovaries to find out what they were doing during the cycles," Pierson says.

The team tracked 50 women with normal menstrual cycles who volunteered to undergo high-resolution ultrasound every day for a month so researchers could follow the fate of every individual follicle.

They found follicles grow in waves "like you see in the ocean," Pierson says.

Forty per cent of women had multiple, major waves, while 60 per cent had minor waves, followed by a major wave.

"We were flabbergasted. We knew this happened in animal models (in horses and cows), but we read the books like everybody else and they said this isn't supposed to happen in humans."

Although the women all released only one egg during the study cycle, "they had the biological machinery to ovulate more than once." Two women with abnormal cycles actually ovulated twice.

"Many women will ovulate two follicles at once, typically on the same day. That's how fraternal twins come about," Pierson explains. "But these women ovulated at different times."

He says more research is needed to determine what causes some women to experience more follicle waves than others, and how long the follicle actually has a viable egg.

In the meantime, "it tells us why some women don't get along well with oral contraceptives." Most birth control pills are based on a 21-day treatment cycle, where women take active hormones, followed by seven days of placebo or "dummy" pills, which trigger menstruation. Another study by the same team in the same issue of Fertility and Sterility suggests women can still get pregnant during their hormone-free interval, because there's enough space in the pill-free period that allows the development of a new wave of follicles.

Some fertility experts say the finding makes an argument for a continuous pill regimen. "It questions the need for women to have a pill-free week," says Dr. Arthur Leader, chief of reproductive medicine of the University of Ottawa.

The research may also explain why some women undergoing in vitro fertilization and other fertility treatments don't respond to ovary-stimulating drugs.

"We're probably giving at least some of the women drugs at the wrong time," Pierson says.