Jean Twenge has a very helpful article in The Atlantic. She counteracts the widespread view, made popular by Sylvia Ann Hewlett, that women's fertility drops significantly in their early 30s. The good news is that better studies based on more modern populations find that most women trying to have children through their 30s will be able to conceive without assistance.
I know that Hewlett's view is widespread because I have been one of those spreading it. Since her Creating a Life: Professional Women and the Quest for Children came out in 2002 I have been teaching her report that women's fertility begins to decline significantly in their late 20s. This tends to produce consternation among young women, as it did Twenge who read Hewlett's report when Twenge was herself 30 and newly divorced.
The new research is not a complete about-face. It is true that women's fertility declines after their 20s. The difference is that the decline is more gradual than the data Hewlett - an economist who relied on other people's studies for this part of the book - had reported.
I think there is another part of this story to consider, though. Hewlett's book was not about all women. She set out to study how the highest achieving women broke the glass ceiling. To her surprise, what many of those women wanted to talk about, instead, was how they missed having as many children as they had wished to, and how a sizable fraction had had no children at all. These high-achieving women did not decide not to have children, but rather found themselves in their 40s and beyond having made a "creeping non-choice" for no kids.
Twenge found that most women in their 30s, and many in their early 40s, can conceive normally. So why were Hewlett's women different?
No one has the data to answer that question right now. Other research, though, suggests a possibility to me: the population of women who make it to the top of competitive professions are more likely to include higher-than-average levels of testosterone. And high-testosterone women may have more trouble conceiving at every stage of life.
It may be, therefore, that Hewlett was not so far off for the population she studied. The error may have been in assuming that all other populations of women would have the same fertility chances.