San Francisco women have children later than anywhere else in the U.S. Here's why.

GALLERY: The average ages women become mothers in Bay Area counties GALLERY: The average ages women become mothers in Bay Area counties Photo: Christina Koci Hernandez, SFC Photo: Christina Koci Hernandez, SFC Image 1 of / 14 Caption Close San Francisco women have children later than anywhere else in the U.S. Here's why. 1 / 14 Back to Gallery

Women in San Francisco County become first-time mothers, on average, at age 31.9. If they're married, that number rises to 32.4, and if they've got a college degree, it increases to 32.9.

The figures come from a recent New York Times report that looks at the ages women enter motherhood and why the number varies from place to place.

The women of San Francisco County become mothers later than anywhere else in the country, the data determined. Manhattan mothers trailed closely behind in the age ranking, as did those in Marin County.

The average age of first-time mothers in the U.S. is 26, and for fathers it's 31. Those numbers have increased with the decades. In 1972, women typically gave birth at age 21.

WHERE HAVE THE CHILDREN GONE: SF has lowest percentage of kids of any major U.S. city

Why are San Francisco women giving birth later than others? The answer, though complex, largely boils down to education, the Times found. Women without college degrees give birth an average of seven years earlier than those with degrees.

"If going to college and achieving an upper-middle-class lifestyle seems unattainable, then having a family might seem like the most accessible source of meaning to you," Caroline Hartnett, a sociologist at the University of South Carolina, told the Times.

College, the report found, is a stronger indication of later entry into motherhood than geography or home prices. Then again, these things are linked.

More than 43 percent of San Francisco-Oakland-Fremont residents held college degrees in 2010, according to data from the Brookings Metropolitan Policy Program. The SF-Oakland-Fremont area placed fourth on Brookings' ranking of cities with the most college-educated residents. San Jose-Sunnyvale-Santa Clara came in second.

There's also the matter of home values. In April, Zillow said the median home value in San Francisco peaked at $1.3 million. In Santa Clara, it was $1.4 million.

Behind housing, child care is often the most significant expense for families, according to the Children's Council San Francisco. The organization estimates it costs up to $29,508 for care for children up to age two, and $22,560 for those between age two and five.

Between child care costs and the high price of real estate, it's a high barrier of entry to comfortably start a family in many parts of the Bay Area.

Women who wait to have children may spend that time attending college or graduate school and advancing in their careers. And with the wage gap what it is, establishing career stability can lead to greater financial payoff in the long run, even after a woman has given birth.

All of these factors help drive social inequality, Heather Rackin, a sociologist at Louisiana State University, told the Times. After all, children of college-educated parents are more likely to attend college themselves.

WHY PEOPLE MOVE: It's not just housing costs pushing people out of the Bay Area

"Lower-socioeconomic-status people might not have as many opportunity costs — and motherhood has these benefits of emotional fulfillment, status in their community and a path to becoming an adult," Rackin said.

Even with its surfeit of college-educated residents and high-paying jobs, San Francisco may be simply too expensive for some families.

A New York Times piece published in 2017 reported San Francisco has the lowest percentage of children of any major U.S. city. Only 13 percent of the city's population is under 18 years old, compared to 21 percent in New York and 23 percent in Chicago.

"As a mother of three kids in San Francisco, I'm familiar with this trend because my years of parenting have been filled with heartbreak over saying goodbye to fleeing families we grew close to through playgroups, preschool, the neighborhood," wrote SFGATE producer Amy Graff of the Times report.

"My heart eventually numbed to the the mass exodus and I now approach new friendships in the way an Army brat might at her third high school," she continued. "I'm eager to meet new people but hesitant to get too close to protect myself from future partings."

Read Michelle Robertson's latest stories and send her news tips at mrobertson@sfchronicle.com.

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