Their finding, if verified, would overturn the traditional wisdom that women produce an egg cell once a month. It would also help explain why "natural" methods of birth control, based on the idea that ovulation can be predicted, often fail.

"We are literally going to have to re-write medical textbooks," said Dr. Roger Pierson, director of the Reproductive Biology Research Unit at the University of Saskatchewan, who led the study.

"It's exactly why the rhythm method doesn't work."

Scientists have long known that humans have unique cycles of ovulation. Many animals come into heat -- a time when all the males around know through smells and visual signals that a female is ovulating and ready to conceive.

Not so with humans, who have "concealed" ovulation.

Standard medical science says a woman has a cycle running roughly 28 days in which an egg ripens, is released by the follicle, drops into the fallopian tube, and then is either fertilized or shed during menstruation.

Writing in the journal Fertility and Sterility, Pierson and colleagues found this did not always happen.

"We weren't expecting this. We really weren't," Pierson said in a telephone interview.

DAILY ULTRASOUND SCANS

In the study, Pierson, veterinarian Gregg Adams and graduate student Angela Baerwald did daily, high-resolution ultrasound scans on 63 women for a month, which allowed them to see the follicles very clearly.

"We had 63 women with normal menstrual cycles. Of those 63, only 50 had normal ovarian cycles," Pierson said.

Thirteen of the women ovulated multiple times, in various different ways. And of the other 50, 40 percent had up to three waves of activity by the follicles, any one of which could result in the production of an egg.

The women's hormone levels did not match this activity, Pierson said. "Hopefully this will help women explain how they got pregnant when they really didn't want to be pregnant, and it certainly will help us design better fertility therapies."

Apparently, measuring hormones in the blood is not enough to predict what a woman's reproductive system is up to.

"The hormones do what they are going to do and the ovaries just follow their merry path," Pierson said.

"We always thought that menstrual cycles and ovarian cycles were one and the same. It turns out they are just like two political parties -- sometimes they go along hand in hand for the good of the country and sometimes they go along their separate ways."

Pierson's team plans longer-term studies to see if the women's patterns are consistent from month to month.

"We don't know what's causing it -- we don't know if it is the weather or exposure to men or grapefruit juice or what," Pierson said.

The findings, which were first seen in cattle and horses, help explain some things that have puzzled obstetricians, Pierson said.

"It really explains how we get fraternal twins with different conception days," Pierson said. "Clinically, we see this all the time. We see women come in with twins and when we do an ultrasound we see one is at one 10 weeks development and another at seven."