Other factors cited by Pew to explain the increase include more immigrant mothers, who tend to stay home with children in greater numbers than U.S.-born moms; more women unable to find work; and ambivalence about the impact of working mothers on young children.
The share of mothers who do not work outside the home rose to 29 percent in 2012, the study found. That's up from 23 percent at the turn of the century, according to the report. At the height of the recession in 2008, Pew estimated 26 percent of mothers were home with children.
The at-home moms include women who are married, single, disabled, enrolled in school or unable to find work.
Pew cited a 2010 U.S. census report that singled out the expense of child care as a factor. In inflation-adjusted dollars, the average weekly child care expense for families with working mothers who paid for child care rose more than 70 percent, from $87 in 1985 to $148 in 2011, according to government estimates. That represented 7.2 percent of the income for such families.
Tricia Williamson, 30, in Liberty, N.C., quit her job as an editor and producer at a TV station after crunching the numbers and realizing her salary after the birth of her son a year ago would go primarily to commuting and child care expenses. Her husband earns about $44,000 a year as an electronics technician.
"We're not rich by any means. We live paycheck to paycheck, but it's completely worth it," she said. "My son wouldn't be getting the attention he needs one-on-one. He's got mom 24-7."
The largest share of at-home mothers - roughly two-thirds of 10.4 million - had working husbands. A growing share - 6 percent in 2012, up from 1 percent in 2000 - said they could not find a job, according to Pew, which relied on U.S. Census and other government data.
No matter what their marital status, mothers at home are younger and less educated than working counterparts, the report said. Most married moms said they were home specifically to care for the kids, while single mothers were more likely to say they couldn't find a job, were ill or disabled, or were in school.
Among all at-home mothers in 2012, 51 percent had at least one child 5 or younger, compared with 41 percent of working mothers.
The researchers said one of the most striking demographic differences between at-home mothers and working mothers is their economic well-being, with about 34 percent of at-home mothers living in poverty, compared with 12 percent of working mothers.
Relatively few married at-home mothers with working husbands qualify as "affluent," at nearly 370,000 with at least a master's degree and a median family income of over $75,000 a year in 2012. That number amounts to 5 percent of married at-home mothers with working husbands.
The "elite" marrieds stand out from other at-home mothers as disproportionately white or Asian. About 69 percent are white and 19 percent are Asian. Only 7 percent are Hispanic and 3 percent are black.
Mothers more likely to stay home are among demographic groups on the rise in the U.S. For example, 40 percent of immigrant mothers were at home with their children, compared with about a quarter of U.S.-born mothers.
Among at-home mothers living in poverty in 2012, 36 percent were immigrants, the report said.
The report points to stagnant incomes for all but the college-educated as a possible factor for less-educated workers in particular who might be weighing the cost of child care against wages and deciding it makes more economic sense to stay home.
While attitudes over the decades toward working mothers have improved, "most Americans continue to believe that it's best for children to have a parent at home," said D'Vera Cohn, a senior writer at Pew who worked on the report.
Since 2008, about 70 percent said when questioned in an ongoing social survey that a working mother is just as capable as an at-home mother of establishing the same "warm and secure" relationship with her children. But 60 percent of Americans in a recent Pew survey said children are better off when a parent stays home to "focus on the family," compared with 35 percent who said children are "just as well off with working parents."