IN 2007 Cathy McMorris Rodgers, a 37-year-old Republican congresswoman from Washington state, became the first woman in over a decade to give birth in office. In 2010 she had a second child, and in 2013 a third. That made her the first, and so far only, woman to give birth more than once while in Congress. Yet although her experience is rare among legislators, new data on births show that she is, in fact, rather typical of one category of American women. While the overall birth rate keeps declining, well-educated women seem to be having more children, not fewer. A new report from the Pew Research Centre, based on an analysis of census data, looks at women who have reached their mid-40s (when the vast majority of women stop having children) over the past two decades. It finds that the proportion of all women who reach that age without ever having a child has fallen, but the decline is sharpest among the best-educated women. In 1994, 35% of women with a doctoral degree aged 40 to 44 were childless; by last year, this had fallen to 20% (see chart). Their families are bigger, too. In 1994, half of women with a master’s degree had had two more or more children. By last year, the figure was 60%. It still holds true that the better-educated a woman is, the less likely she is to have a child. But Pew’s data show that the gap has narrowed.

At the same time, data from the Centres for Disease Control, which monitors births, show that the average new mother has become substantially older. Since the start of recession in 2007, America’s general fertility rate has declined to its lowest level in recorded history. But all that drop is accounted for by teenagers and women in their 20s. Birth rates for women in their 30s have increased markedly. Because older, better-educated women are more likely to be married, this has also stabilised the proportion of children born in wedlock. In 2009 41% of newborn babies had unmarried parents; in 2013, 40.6% did.

Why might older, better-educated women be having more children? Partly because access to education has widened—and so women who were always going to have children are spending more time in college. Another reason is that fertility treatment has improved dramatically, and access to that, too, has widened. Older women who, in the past, wanted children but were unable to have them are now able to.

But according to Philip Cohen, a demographer at the University of Maryland, this does not explain the entire leap. Rather, social changes in the nature of marriage seem to be driving the change. Whereas marriage was once near-universal and unequal, in recent decades it has become a deliberate option and more equal. Well-educated women have been able to form strong relationships with similarly brainy men, in which both parents earn and both do some child care. Getting an education and having a career are no longer always a barrier to having children; sometimes, they make it easier.

In 1965, mothers spent seven times as long caring for children as fathers did. By 2012 they were spending “only” twice as much time elbow-deep in formula and Pampers. According to Stephanie Coontz, of Evergreen State College in Washington, how much time fathers spend helping to bring up children is the main determinant of whether mothers will have a second child. And as more career-minded women have had children, they have become powerful enough to demand time off from their employers. Although America has no national system of paid maternity leave, many professional firms now offer it—Ernst & Young, an accountancy firm, offers 39 weeks to its employees, for example.

Poorer women, however, have had little luck of that sort. As the economic prospects of blue-collar men have faded, women find the menfolk are pitching in not more, but less, suggests Ms Coontz: “And if I’m a lower-income woman…do I want to hitch myself to a guy who may become just another mouth to feed?” Their employers, meanwhile, are less likely to give them time off. When recession came in 2007, it was these women who delayed having children. Giving birth is not yet a luxury good—but it may be getting that way.