Two posts at Crooked Timber worth reading: on the childlessness of women vs. men academics; on the increasing duration of the professorial track in recent decades.
Off-the-cuff reaction #1: If you're a woman academic who wants to have children and get on the tenure track at a competitive research university, you should look for a man who's likely to stay home and take care of the kids. With all that entails. Unfortunately, most women's libidos are wired by culture to respond most strongly to men who are at least somewhat aggressive, ambitious, and dominating. Alas, these traits do not correlate strongly with the tendency to become a stay-at-home husband.
(Off-the-cuff reaction #1.5: Perhaps women who find their libidos wired in this tragic fashion could resolve this conundrum the way that men have traditionally resolved the converse conundrum. Let the sensitive house-husband raise the kids; cheat on him periodically with aggressive, dominating men. In case the "converse conundrum" for men is not obvious: most men are wired, by culture, to respond to perky taut-bodied young women who hang on their every word, which does not usually describe a wife after she's had a few children. The traditional male solution has been to cheat on the wife with a younger woman. I leave it to the reader to puzzle out whether I am seriously suggesting that women academics cheat on their model-father husbands, or whether I am suggesting that women academics should reconcile their libidos with their circumstances. I am not sure which of these suggestions is more offensive, but they seem like the obvious alternatives. Of course, the statistics tell us that most women academics are not having children, and hence probably not married to homemaker Dads, so this hypothetical situation isn't exactly representative of reality anyway.)
Off-the-cuff reaction #2: The inescapable conclusion of the second CT post is that, because of the asymmetry in male vs. female fertility, the current academic system is structurally biased towards men. The 22-34 window, during which you have to get your Ph.D. and then immediately work your ass off to get tenure, overlaps with most of a woman's prime childbearing years. This sounds utterly obvious but it has never before hit me with the same force.
We should move to a system in which junior faculty can take parental leave for a few years, sometime before the tenure review. Or, we should remove the stigma attached to getting your Ph.D. in your 30's instead of your 20's; and make it easier for grad students to take a few years off from the program after quals. Or all of the above. These reforms would have major benefits for men as well: not everyone fits into the classic 12-year career path anyway.