It has become harder for women to work and have children.

The cost of motherhood fell for most of the 20th century because of inventions like dishwashers, formula and the birth control pill. But that’s no longer the case. The cost of child care has risen by 65 percent since the early 1980s. Eighty percent of women breast-feed, up from about half. And the number of hours that parents spend on child care has risen, especially for college-educated parents, for whom it has doubled.

These are some of the factors researchers uncovered when they set out to solve the mystery of why more women aren’t working. Most women plan to work, the researchers found, but are increasingly caught off guard by the time and effort it takes to raise children.

The share of women in the United States labor force has leveled off since the 1990s, after steadily climbing for half a century. Today, the share of women 25 to 54 who work is about the same as it was in 1995, even though in the intervening decades, women have been earning more college degrees than men, entering jobs previously closed to them and delaying marriage and childbirth.

The new analysis suggests something else also began happening during the 1990s: Motherhood became more demanding. Parents now spend more time and money on child care. They feel more pressure to breast-feed, to do enriching activities with their children and to provide close supervision.

A result is that women underestimate the costs of motherhood. The mismatch is biggest for those with college degrees, who invest in an education and expect to maintain a career, wrote study authors Ilyana Kuziemko and Jenny Shen of Princeton, Jessica Pan of the National University of Singapore and Ebonya Washington of Yale.

The study — a working paper, meaning it has not yet been published in a peer-reviewed journal — documented a sharp decline in employment for women after their first children were born, in both the United States and Britain, even though about 90 percent of the women worked before having children. They used data from the Labor Department’s National Longitudinal Surveys, the University of Michigan’s Panel Study of Income Dynamics and the British Household Panel Survey. Each covers several decades, but the study focused mostly on women born between 1965 and 1975, meaning they were in their 30s in the 2000s.

A change of plans

For many women, the researchers show, stopping work was unplanned. Since about 1985, no more than 2 percent of female high school seniors said they planned to be “homemakers” even though most planned to be mothers. The surveys also found no decline in overall job satisfaction post-baby. Yet consistently, between 15 and 18 percent of women have stayed home.

One key to understanding why women have diverged from their plans, the economists found, is that their beliefs about gender roles change after their first baby. The surveys ask questions like whether work inhibits a woman’s ability to be a good mother and whether both parents should contribute financially to a family. Women tend to give more traditional answers after becoming mothers. (Women still do the bulk of child care, even in two-earner families.)

The people most surprised by the demands of motherhood were those the researchers least expected: women with college degrees, those who had babies later, those who had working mothers and those who had assumed they would have careers. Even though highly educated mothers were less likely to quit working than less-educated mothers, they were more likely to express anti-work beliefs and to say that being a parent was harder than they expected.

Though the study did not analyze fathers’ role in depth, it found that their beliefs did not change significantly with the arrival of a baby. They were less likely than women to say that parenthood was harder than they expected.

Work-family conflict

Overall, women have had great success in entering the labor force. Seventy percent of mothers with children under 18 work. Women are more likely to work than previous generations at almost every age, found Claudia Goldin, a Harvard economist. That just adds to the mystery of why they’re not working as much.

“It is deeply puzzling that at a moment when women are more prepared than ever for long careers in the labor market, norms would change in a manner that encourages them to spend more time at home,” the researchers wrote.

One possible reason is that increasingly, people who work long, inflexible hours are paid disproportionately more, Goldin’s research has found. More women with degrees and these kinds of demanding jobs are having children, and they’re likely to be married to men with similar jobs, as Marianne Bertrand, an economist at the University of Chicago, has described.

A result is that dual-earning couples may feel the best choice is for one member, usually the mother, to step back from work so the other parent can maximize the family’s earnings.

As women do more paid work, men have not increased their child care and housekeeping tasks to the same extent — another surprise for young women who, research has shown, expected more egalitarian partnerships.

Generations of girls have been told they can have both a career and children. But both work and parenting have become more demanding. The result is that women’s expectations seem to be outpacing the realities of public policy, workplace culture and family life.