Better educated Australian women have fewer children than those with no post-school qualifications, researchers have found.
Going to university also reduces a woman's likelihood of marriage, they say. Whereas a large number of early school-leavers become mothers in their twenties, female students are busy studying, graduating and developing careers.
By the time women start thinking of having a child, the men they might have married are not available. Even among those who have children, the proportion having two or more drops with increasing education levels.
Those with no post-school qualifications have an average of 2.3 children; those with a bachelors degree (about one in six of the Australian female population) have an average of 1.8; and those with a higher degree have 1.3 children. The implication is that for every two additional qualifications a woman has one fewer child.
James Franklin and Sarah Chee Tueno's research also found a similar pattern for childless women. Nearly 90 per cent of women with no post-school qualifications have children. This compares with 78 per cent of bachelors-degree holders who become pregnant and 66 per cent of those with a higher degree.
The situation facing well-educated women is not confined to Australia: Singapore tried to encourage its female graduates to have more children, but had little success. In other Western countries, lower fertility of graduates is widespread, the researchers say.
US studies have similarly found that the more successful a woman is in her career, the less likely she is to find a partner or bear a child. A third of high-achieving American women are childless at 40, as are nearly half those earning more than $100,000 (£56,000) a year.
Professor Franklin and Ms Tueno noticed a strong "marrying up" phenomenon, with women tending to form relationships with men who are better educated.
"This leads to a large excess of unpartnered educated women. By the time they are in their forties, the ratio of women graduates without partners compared with men is three to two."
Professor Franklin is a mathematician at the University of New South Wales, where Sarah Chee Tueno graduated.
They say that well-educated women do not avoid having children, but rather, the circumstances they find themselves in make it difficult. Study fills the most fertile years for female students and new graduates. Then they need time to establish their careers, the researchers say. But infertility increases with age, so for some childbearing is not simply postponed but ruled out.
The researchers note that the transition from university to professional work means long hours on the job. Women who take time off to care for babies must often depend on one income from a partner rather than two, and a return to work may be uncertain.
"The difficulties of time, money and insecurity are compounded by problems in finding a suitable partner," the researchers say. "They are magnified by the enduring tendency of women to marry up. It can be more difficult for women graduates to find husbands than for non-graduates."
To tackle this, governments should cut the financial risk of childbearing by cancelling tuition debts and paying a "living wage" to postgraduate researchers.
Professor Franklin and Ms Tueno say that childlessness is not a deliberate choice among female graduates - more than 90 per cent of young women say they would like to have children by the time they are 35. But as they grow older their expectations fall: during the ages of 20-24, those with post-school qualifications expect to have 2.55 children on average. This drops to 1.81 children by the time they are in their early thirties - still "much more than is likely to be achieved".
"Education takes time, graduate work takes time, forming and sustaining relationships takes time, and motherhood takes time," the researchers write. "There is only so much time to go around."