Guidelines for getting pregnant -- or avoiding pregnancy -- usually assume an average woman is fertile between days 10 and 17 of her menstrual cycle. But researchers at the National Institute of Environmental Health Sciences have demonstrated what some accidentally pregnant women may have long suspected: Only about 30 percent of women actually have their fertile window entirely within that timespan.

In fact, the researchers found, there is hardly a day in the menstrual cycle during which some women are not potentially fertile. Women in this study were of prime reproductive age (most between 25 and 35), when the menstrual cycles are most regular. Because teenagers and women approaching menopause have less regular cycles, their windows of fertility might be even more unpredictable.

Describing data from nearly 700 menstrual cycles of 213 healthy North Carolina women, NIEHS' Allen J. Wilcox, M.D., Ph.D., statistician David Dunson, Ph.D., and epidemiologist Donna Day Baird, Ph.D., reported in the British Medical Journal (Vol. 321, pp 1259-1262) that the timing of women's fertile window "can be highly unpredictable, even if their cycles are usually regular."

Dr. Wilcox, a physician and chief of epidemiology at NIEHS, said, "If the average healthy couple wants to get pregnant, they are just as well off to forget 'fertile windows' and simply engage in unprotected intercourse two or three times a week." As for those seeking to use the cycle to avoid pregnancy, the odds are not good. "Our data suggest there are few days of the menstrual cycle during which some women are not potentially capable of becoming pregnant - including even the day on which they may expect their next menses to begin," according to the new scientific report.

With cycle day one being the day of the actual onset of menstrual bleeding, the researchers showed that 2 percent of women were starting their fertile window by day four, 17 percent by day seven. More tha
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Contact: Bill Grigg
grigg@niehs.nih.gov
301-402-3378
NIH/National Institute of Environmental Health Sciences
16-Nov-2000