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Birth Control Health Center

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Modern Rhythm

Finding 'Safe' Sex Days

Numbers Remain Small continued...

In fact, menstrual cycles can vary from one woman to the next, and for many women, from one month to the next. Stress or illness, for instance, can disrupt even the most regular cycles. Such inherent variability was recently demonstrated in a study of 221 healthy women, published in the British Medical Journal in November 2000. Using daily urine tests to check for hormonal evidence of ovulation, researchers from the National Institute of Environmental Health Sciences found that even though clinical guidelines assume the average woman is fertile between days 10 and 17 of her menstrual cycle, only 30% of the women studied had their window of fertility fall entirely within that time period. Even women with reportedly regular cycles had a 10% chance of being fertile "on any given day of their cycle between days six and 21," the researchers wrote.

"What was surprising to us is the fact that not only were fertile days coming early in the cycle, but late when a woman thinks she's on the end of her cycle," says Allen J. Wilcox, MD, PhD, chief of epidemiology at the NIEHS and lead author of the study. "We're just putting numbers on something people had a sense of before."

The researchers also point out that most of the women in the study were between the ages of 25 and 35. Teenagers and women nearing menopause tend to have even more unpredictable cycles.

It's Not Guesswork

Homan calls the calendar rhythm method "a guessing game, pure and simple," but emphasizes that there is more to natural family planning than just counting days. The more modern variations rely upon physiological signs such as changes in cervical discharge, body temperature, cervix position, or if it's the "sympto-thermal" method, a combination of all three, to signal whether a woman is fertile. "Modern natural family planning doesn't try to predict anything," he says. "It's, 'What you see is what you are.'"

Using these indicators, he says, a woman should be able to tell when she is in the preovulation, fertile, or postfertile phase of her cycle. Couples attempting to avoid pregnancy either can abstain from sex during the fertile phase or use other forms of protection.

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