From Jean Twenge's "How Long Can You Wait to Have a Baby?" in The Atlantic this month:
The widely cited statistic that one in three women ages 35 to 39 will not be pregnant after a year of trying, for instance, is based on an article published in 2004 in the journal Human Reproduction. Rarely mentioned is the source of the data: French birth records from 1670 to 1830. The chance of remaining childless—30 percent—was also calculated based on historical populations.
In other words, millions of women are being told when to get pregnant based on statistics from a time before electricity, antibiotics, or fertility treatment. Most people assume these numbers are based on large, well-conducted studies of modern women, but they are not. When I mention this to friends and associates, by far the most common reaction is: “No … No way. Really?”