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Women's fertile days are highly unpredictable

The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study

A woman's "fertile window" - six days during her menstrual cycle when intercourse can result in pregnancy - is strikingly unpredictable in any given cycle, according to a study in this week's BMJ. The usual clinical guidelines for getting pregnant or avoiding pregnancy do not take this natural variability into account.

Wilcox and colleagues analysed the menstrual cycles of 213 healthy women who were planning a pregnancy. They found that only 30% of women had their fertile window entirely within the days usually identified by clinical guidelines - that is, between days 10 and 17 of the menstrual cycle. On average, at least 10% of women with regular cycles were in their fertile window on any given day between days 6 and 21. Even on the day their next period was expected, women had a 1-6% probability of being fertile, they add.

The timing of the fertile window is even less predictable for women with less regular cycles, such as adolescents, stress the authors. Women should be aware that no calendar method for identifying fertile days is completely effective, they conclude.

Contact:

Allen Wilcox, National Institute of Environmental Health Sciences, Durham, USA
Email: wilcox@niehs.nih.gov

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