|Medical textbooks say that an average woman is most fertile from 10 to 17 days after the first day of her menstrual cycle. But NIH researchers have shown what some women have long suspected: many women who think they can only become pregnant during this span become unexpectedly pregnant.
For some women with irregular cycles, there is hardly a day in the menstrual cycle when they are NOT potentially fertile. The window of fertility is most unpredictable for teenagers and women approaching menopause.
Textbooks also usually say that women are fertile for several days after ovulation. Dr. Allen Wilcox of NIH's National Institute of Environmental Health Sciences (NIEHS) has studied fertility extensively. “The basic problem is that ovulation is so unpredictable,” he says. His fertility research has shown that women are fertile only on the day they ovulate and the five days prior to ovulation, not at all after ovulation.
“On average it occurs around day 15,” he says, “but it can happen anywhere from day 8 to day 60 or later.”
Most women don't know when they ovulate, however, and that's where it gets tricky. While women are likely to be fertile between days 10 and 17 of their cycle, 70 percent of women have at least one of their fertile days outside that window, he explains.