Is The Traditionally Accepted Model Of The Human Menstrual Cycle Wrong?
"This collaborative discovery is an important step forward in understanding the human menstrual cycle," said Dr Michael Kramer, Scientific Director of the Institute of Human Development and Child and Youth Health of the Canadian Institutes of Health Research. "It provides a new model for ovarian function during the menstrual cycle which could have profound implications for infertility diagnoses and treatment in women."
The team's findings were funded by the Canadian Institutes of Health Research (CIHR) and published in the July 6th, 2003 issue of the scientific journal Fertility and Sterility. A companion paper published in Biology of Reproduction is available online.
Previous research has shown that a group of 15 to 20 follicles grew during the menstrual cycle, and that one follicle from the group was selected to ovulate while the others died off.
Catching a wave
University of Saskatchewan researchers have found that this process occurs in "waves." In response to hormone surges, women experience two to three periods of follicular development each month, though only one egg is selected for ovulation.
"This work is particularly exciting to us because of the impact it will have on women taking oral contraceptives and undergoing fertility treatment," says Dr Roger Pierson, Director of the Reproductive Biology Research Unit at the U of S. "It also shows that we have not fully understood the basic biological processes that occur during menstrual cycles. We are literally going to have to re-write medical textbooks."
For instance, up to 40 percent of women may not be able to use natural family planning methods, he said. That's because for women who experience two or three waves of dominant follicle growth per month there is no "safe" time to have intercourse during the cycle -- there may always be a follicle capable of ovulating.
About the study
The study involved 63 women with normal menstrual cycles who underwent ultrasound every day for a month. "This study is a real tribute to the altruism of Saskatchewan women," Dr Pierson said. "As I've gone around the country talking about this work, people just can't believe the dedication of our research volunteers."
Dr Pierson says further research is needed to see if the same number of waves occurs consistently every month and to determine why a particular egg is selected to ovulate.
Other team members are Angela Baerwald, the lead author on the article who just received her PhD in clinical reproductive biology from the U of S, and veterinarian Dr Gregg Adams, a professor with the Western College of Veterinary Medicine at the University of Saskatchewan.
But we don't say "moo"
The study was done through an unusual collaboration. In clinical studies, Dr Pierson noticed follicular development occurring at a time when the textbooks said it shouldn't happen. He consulted Dr Adams, who developed the follicular wave model in cows with Dr Pierson at the University of Wisconsin in the 1980s.
"What Roger was seeing in women seemed very similar to what is happening in horses and cows that develop follicles in waves," said Dr Adams. "And that's exactly what we found -- that humans are not so very different from other species."