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If you're trying to get pregnant, your calendar is probably filled with all sorts of dates and times. But is your "fertility window" as predictable as you think? Not necessarily, say researchers at the National Institute of Environmental Health Sciences (NIEHS) in Durham, N.C.

Traditional wisdom holds that there are six days in the middle of the menstrual cycle when intercourse can result in pregnancy. Yet a new study, published in the British Medical Journal, found that late ovulation was a common occurrence and that some women are actually fertile throughout their entire menstrual cycle.

Researchers analyzed daily urine samples from 213 healthy women pages 25 to 35, who were planning to get pregnant. The participants also kept a diary that included, among other information, when and how often they had sex. Lead researcher Allen J. Wilcox, M.D., noted that it took several years to analyze the 30,000 urine samples and diaries.

"In one sense," says Wilcox, "what we're describing is how much variability there is in individual women--how long the cycle lasts and when ovulation occurs." In the case of this study, the earliest onset of ovulation occurred on day eight (day one being the actual start of menstruation); the latest was day 60. According to the researchers, the data suggest that there are few days in women's cycles during which they are not potentially fertile, including the day they expect their period to begin. The study did note, however, that timing isn't everything. Pregnancy also depends on the viability of the sperm and egg, the receptivity of the uterus and other factors.

So what does this mean to women who are trying to conceive? Wilcox suggests forgetting about fertile windows. "Most couples who are trying to conceive will have sex up to five or six times a week. [With that frequency], most will conceive within three to four cycles," he says. "It's best to let nature take its course."

The corollary is that couples using the rhythm method as birth control might be in for a shock. Cautions Wilcox: "Women who are more casual because their periods are fairly regular might be surprised to know they're fertile on days they don't expect."