Already the black sheep of birth control, the rhythm method--which is based on predicting a womans menstrual cycle accurately--may be even more unreliable than previously thought. Statistically, it fails in about 20 percent of all cases, making it one of the least effective ways to prevent pregnancy practiced today. Some advocates have blamed much of that unreliability on improper practice of the method. But a study published in last weeks issue of the British Medical Journal, has found that the methods main flaw lies in the menstrual cycle itself.

In the past, clinical guidelines stated that most women were potentially fertile between the 10th and 17th day of their menstrual cycle. This so-called fertile window supposedly begins five days before, and ends with, ovulation. The rhythm method requires knowing when these six fertile days will occur, and abstaining from intercourse during that time.

Researchers at the National Institute of Environmental Health Sciences led by Allen Wilcox, however, found that the fertile window is far more unpredictable than previously assumed, even among women who have very regular menstrual cycles. The study looked at 213 healthy women who were planning a pregnancy and found that in only 30 percent of the cases did their fertile window fit exactly during the period predicted by the clinical guidelines. At least 10 percent of them were fertile on any given day between the 6th and 21st day of the menstrual cycle, and as many as 6 percent were even fertile on the day their next period was expected.

"There are few days of the menstrual cycle during which some women are not potentially fertile," the authors write. In other words, the rhythm method was never really about using science to prevent pregnancy; it was all about luck.