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
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
They found follicles grow in waves "like you see in the ocean,"
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
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
"We're probably giving at least some of the women drugs at the
wrong time," Pierson says.