Menstrual Cycle Proves More Complex

Study Shows Why Rhythm Method Doesn't Work for All

July 10, 2003 -- New research may help explain why the rhythm method fails so many couples and offers further evidence that the monthly window of fertility may vary from woman to woman.

The findings suggest that family planning methods that rely on the calendar just don't work for a significant percentage of women. Investigator Roger A. Pierson, PhD, says roughly one in three women appear to have more than one period of time during the menstrual cycle in which they could potentially ovulate.

In the traditional model of ovulation, a woman with a typical 28-day cycle will ovulate anywhere from 10 to 14 days after the start of her last period, and only one follicle releases an egg. But Pierson and University of Saskatchewan colleagues Gregg Adams, DVM, and Angela Baerwald, BSc, found that most women in their study had at least two times during their cycle when eggs developed (but were not released), and some women had three.

They also found that the timing of ovulation in many of the women did not follow the typical predicted course of 10-14 days after a menstrual period.

"The follicles are not doing what the model says they should, so we have to change the model," Pierson tells WebMD.

Ovulation occurs when an egg is released from a follicle that develops in the ovaries. Hormones trigger the egg's release from a mature follicle.

In the study, 50 healthy, reproductive-age women submitted to daily ultrasound testing for a full menstrual cycle to record changes in follicle development. The researchers found that 68% of the women exhibited two waves of follicle development during their cycle, and 32% exhibited three waves. None of the women had only one phase of follicle development. The findings are published in the July issue of the journal Fertility and Sterility.

The University of Saskatchewan study is not the first to show that the long accepted ovulation model does not work for many women. A National Institute of Environmental Health Sciences study published in 2000 concluded that only about 30% of women consistently ovulate on the days the model says they should. Researcher Allen J. Wilcox, MD, Donna Day Baird, PhD, and colleagues found that sporadic late ovulation was common in a group of 213 women whose ovulation hormone levels were tracked. They found that fertility could occur on almost any day of a menstrual cycle.

"We found that the follicular phase (the days preceding ovulation) leading up to ovulation varied widely," Baird tells WebMD. "That is why the rhythm method doesn't work very well for a lot of women, and it probably doesn't work well for anyone who doesn't abstain from sex a lot. If your window of abstinence is big then you may be OK."

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SOURCES: Roger A. Pierson, MS, PhD, University of Saskatchewan, Canada. Donna Day Baird, epidemiologist, National Institute of Environmental Health Sciences, Durham, N.C.
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