The Seattle area is among a handful of places at the forefront of the shift toward older motherhood. In King County, it's now more common for a woman to have her first child in her 30s or 40s than in her teens or 20s.

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Many women in the U.S. are waiting longer to have children, pushing the average age of first-time moms to an all-time high. But this trend is much more pronounced in some parts of the country than in others.

The Seattle area is among a handful of places at the forefront of the shift toward older motherhood.

In King County, it’s now more common for a woman to have her first child in her 30s or 40s than in her teens or 20s. Fifty-seven percent of first-time mothers in the county were 30 or older, analysis of 2016 data from the Centers for Disease Control (CDC) shows.

How exceptional is that? Very, if you compare King County with the U.S. as a whole. Nationally, slightly less than one-third of first-time moms are 30 or older. The average age for a woman in the U.S. to have a first child was 26.7 in 2016.

Edith Cheng, a professor in the Department of Obstetrics and Gynecology at the University of Washington, says she’s observed this shift toward older moms in her 30 years of practice.

“It’s not uncommon for me to have patients come in and say, ‘Well, I want to have my first child in my early 30s, and I want to be finished by the time I’m 37.’ ”

Cheng thinks that there’s a societal acceptance for women to first have children in their 30s now that didn’t exist when she first entered the field.

Among the 100 most populous counties in the U.S., there are just 20 where, like King, women in their 30s and 40s represent the majority of first-time moms. All 20 counties are located in one of seven metropolitan areas: San Francisco, New York, Boston, Washington, D.C., Portland, Denver and, of course, Seattle.

If it sounds like a familiar list, it’s probably because these metros often make top-10 rankings for things like highest income, most-educated residents and absurdly high rents and home prices.

San Francisco represents the extreme. There, a remarkable 76 percent of first-time mothers are in their 30s or older, which is the highest percentage among the 100 biggest counties (San Francisco is both a city and a county). King County ranks seventh.

“Look at the median income, look at the education level,” Cheng said. “Women tend to be older moms because of work. There’s a delay in having children until they’re a little more established. That’s a very common situation that we’re seeing now.”

In many other parts of the U.S., women tend to become mothers at an age that’s significantly younger than the national average.

El Paso County, Texas, is at the opposite end of the spectrum from San Francisco. Its first-time moms are the youngest among large counties — 85 percent are below age 30. It’s also a much poorer place. Household income there is well below the U.S. median, and 23 percent of the population was below the poverty line in 2016 (by comparison, 9 percent were below the poverty line in King County).

The other counties where women become moms at a young age also tend to have lower incomes and higher rates of poverty.

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A recent New York Times article described the geographic gap in the age that women have babies as symptomatic of the inequality that divides the nation. Women from a higher socioeconomic class often put motherhood on hold to pursue college and career — opportunities that may not be available to women from a poorer background.

We see that divide even within King County. In 2016, the average age of a first-time mom who hadn’t finished high school was 22, according to CDC data. For a first-time mom with a doctorate or professional degree, the average age was just shy of 35.

The rise in the age of first-time mothers in King County has been remarkably swift. From 2007 to 2016 — just nine years — the share of first-time mothers who were 30 or older jumped from 45 percent to 57 percent.

Nationally, the Great Recession intensified the shift toward later motherhood, according to a report by the Pew Research Center. And in King County, you do see an acceleration of the trend around 2009.

But even as the Seattle-area economy picked up steam toward the beginning of this decade, the age of first-time mothers continued on its upward trend. The year 2013 marked the first time that the majority of King County women having their first child were age 30 or older.

One big factor behind the rising average age of first-time moms in the U.S. is that there are fewer teenage women having babies. We see this in King County, too.

In 2008, more than 1,000 teenagers in King County gave birth to their first child. In 2016, only 422 did.

Cheng says there are some increased health risks associated with pregnancy after 35, such as hypertension and diabetes, and those are things she would watch for in a patient. But in the absence of those common conditions of aging, she says she would expect a healthy woman over 35 to have a relatively uncomplicated pregnancy.

“This idea of being old at 37 is no longer there … women are exercising, they’re more aware of their well-being,” Cheng said. “Women in their 30s and early 40s are, as a group, healthier this generation than my generation — I’m 62 — and, clearly, my mother’s generation.”