The number of women in the workforce has fallen to levels not seen in nearly 30 years.

The labor force participation rate of women aged 16 or over – those who are either employed or actively seeking work – hit 56.7% in September, down from 56.9% in August, according to data released by the Bureau of Labor Statistics Friday. It hasn’t been at those levels since 1988.

The rate began climbing when women started to enter the workforce in the 1950s and, after a half-century of growth, eventually peaked at 60.3% in April 2000.

“Women have begun to depart the labor force in a way that’s similar to men,” says Mark Hamrick, Washington, D.C. bureau chief of personal finance website Bankrate.com. But he says it’s not just because of economic factors. The ratio of job losses due to the recession among men versus women were 2.6 to 1, in part because women held more jobs in less cyclical, services-related industries. (Male labor participation was 69.1% in September for men aged 16 years and older, down from 69.2% in August.)

The participation rate of women was essentially flat from the mid-1990s through the Great Recession, which represented a long period of economic expansion in the U.S., says Rakesh Kochhar, associate director of research at the Pew Research Center. “All economic models that take demographics into account project an ongoing decline of women’s participation in the labor force,” he says. The increase in the past was largely due to baby boomer women, he adds, “and they are ageing out of the labor force.”

See also: Stay-at-home moms: Leaning in or left out?

Another reason the employment rate could be lower is that younger women are staying in college or graduate school longer, says Mitra Toossi, economist at the Bureau of Labor Statistics. Women’s participation rate fell from 63% in 2000 to 53.5% in 2013 for those ages 16 to 24. They’re continuing with their education as long as possible until the economy improves, she says, and the unluckier ones have gotten laid off. “During a recession, the young people are the last to be hired and the first to be fired and facing competition from more senior employees,” she says.

Also see: 40% of unemployed workers are millennials

The number of stay-at-home moms rose to 29% of mothers in 2012, up from a modern-era low of 23% in 1999, although still down from 49% over four decades ago, according to an analysis of government data by Pew, a U-turn on the long-term decline in the number of stay-at-home moms. The category includes mothers who choose to stay at home to care for their families, but also those who are at home because they are unable to find work, are disabled or are enrolled in school.

One reason for the rise in stay-at-home motherhood: Childcare is the biggest household expense in most regions, says Lynette Fraga, executive director of Child Care Aware of America, a non-profit group in Arlington, Va. that works with state and local agencies. The average cost for full-time care for an infant ranges from $4,863 a year in Mississippi to $16,430 in Massachusetts, according to its annual report. The U.S. is one of the few industrialized nations that doesn’t require paid family leave for new parents.

And women don’t always want to make that choice between a career and motherhood. Studies show women want bigger families. Women in the U.S. have an average of 2.1 children (versus their ideal rate of 2.4 children), according to an analysis ofgovernment data by Pew Research Center. And 40% of U.S. women nearing the end of their childbearing years say they have fewer children than their ideal, according to the General Social Survey carried out by NORC at the University of Chicago, a non-partisan, independent research organization.

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