at The Atlantic:
…As a psychology researcher who’d published articles in scientific journals, some covered in the popular press, I knew that many scientific findings differ significantly from what the public hears about them. Soon after my second wedding, I decided to go to the source: I scoured medical-research databases, and quickly learned that the statistics on women’s age and fertility—used by many to make decisions about relationships, careers, and when to have children—were one of the more spectacular examples of the mainstream media’s failure to correctly report on and interpret scientific research.
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. …
The data, imperfect as they are, suggest two conclusions. No. 1: fertility declines with age. No. 2, and much more relevant: the vast majority of women in their late 30s will be able to get pregnant on their own. The bottom line for women, in my view, is: plan to have your last child by the time you turn 40. Beyond that, you’re rolling the dice, though they may still come up in your favor. “Fertility is relatively stable until the late 30s, with the inflection point somewhere around 38 or 39,” Steiner told me. “Women in their early 30s can think about years, but in their late 30s, they need to be thinking about months.” That’s also why many experts advise that women older than 35 should see a fertility specialist if they haven’t conceived after six months—particularly if it’s been six months of sex during fertile times.