Couples relying on the "rhythm method" of birth control may want to throw away their calendars. Those hoping to conceive by focusing their reproductive efforts on certain days of the month might end up doing the same. Why? As many as 70% of women are fertile either well before or after the time when today's textbooks say they should be able to conceive.
For decades, doctors have advised women that they are most fertile between days 10 and 17 of a typical 28-day menstrual cycle, in which day 1 is the first day of a woman's period. This guideline was based on studies that suggested women ovulate about 14 days into their cycle and are fertile for a few days before and after ovulation.
Those estimates are wrong, says epidemiologist Allen Wilcox of the National Institute of Environmental Health Sciences in Durham, North Carolina. Wilcox and his colleagues tracked the fertility of 213 healthy North Carolina women between 24 to 35 years old who were planning to get pregnant. The researchers analyzed daily urine samples for the telltale shifts in hormone levels that signal ovulation. The team then compared the ovulation records with the women's diaries of menstruation days and sexual activity.
After analyzing 696 menstrual cycles--during which 62% of the women got pregnant--the researchers found that the window of fertility isn't fixed. Only 30% of women were fertile exclusively during the predicted week. At least 10% of women with regular cycles were fertile between days 6 and 21, and as many as 6% were fertile even on the last day of their cycle, Wilcox's team reports in the 18 November British Medical Journal. The fertile period spans the day of ovulation and the 5 days before, they found, and ovulation isn't nearly as predictable as people thought, even in women with regularly timed menstrual cycles.
"These results throw out a lot of folklore," says biostatistician Louise Ryan of Harvard University. Couples watching the calendar to time conception-free sex should be cautious, she says. Because fertility peaks before the first signs of ovulation, there's no way to know whether any given day falls within a woman's fertile window.