In a recent article in The Atlantic, Jean Twenge investigated the popularly held belief that women's fertility takes a nosedive at 35. What she found revealed as much about our understanding of statistics as it did about fertility.
Let's just start with that "5 percent at 40" statistic. Analysis by David B. Dunson of Duke University's Department of Statistical Science showed that there was an age-related decline in female fertility, but not at the precipice-like drop older studies had shown. His research showed that intercourse during the fertile period resulted in pregnancy 29 percent of the time for 35-to-39-year-old women, which is about a third less than that of 27-to-29-year-olds, who will conceive 42 percent of the time.
If this is true, then where does that scary, 5 percent stat come from? The short answer is, no one knows.
"...no journal article I could locate contained these numbers, and none of the experts I contacted could tell me what data set they were based on," Twenge wrote. "The American Society for Reproductive Medicine's guide provides no citation for these statistics; when I contacted the association's press office asking where they came from, a representative said they were simplified for a popular audience, and did not provide a specific citation."
Dunson told Twenge that he thought the difference could be because of averaging across many cycles of trying to get pregnant, rather than using the first cycle. The problem with doing this is that most fertile women will get pregnant quickly when they start trying, and averaging across cycles means that we inevitablely include in our sample women and men with fertility problems that may or may not be related to age. His own study showed a 4 percent drop in fertility between the ages of 28 and 37. Which, as Twenge points out, means that most women who try to conceive over the age of 35 will be able to do so.
One last statistic before we leave you to read the full article, which should be required reading for all women who are pondering leaning out before their chance at motherhood slips away. Researchers love to quote a stat from a 2004 article in the journal Human Reproduction, which stated that one in three women aged 35 to 39 wouldn't get pregnant after a year of trying. The wrinkle? Those statistics are based on French birth records from 1670 to 1830.
Or, as Twenge puts it:
"We've rearranged our lives, worried endlessly, and forgone countless career opportunities based on a few statistics about women who resided in thatched-roof huts and never saw a lightbulb."
In other words, things have changed slightly since 1700s, and while no one would suggest that you wait until the very last minute to have a baby, neither can anyone tell you, with any preciseness, when the last minute is. The lesson for women who want to have a career and a family is to be a skeptical consumer of information ... and remember that no one, not even biostatisticians, can predict your life down to every detail.
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