After 10 years of living in the District, Zach Lipson and his wife, Stacey, moved to Bethesda to have a baby. They now have an 11-week-old daughter, Elle. (Jonathan Newton/The Washington Post)

In a development that caught some local leaders by surprise, the District of Columbia is one in just a handful of urban centers in the United States that did not enjoy gains in median household income in 2016, according to Census Bureau statistics released Thursday.

The slight decline — from $76,233 to $75,506 — followed at least eight years of income growth as the city became a magnet for young professionals across the country.

Theories for the plateau abound. Among them are the city's lack of affordability for even relatively high-earning families and a drop in the number of high-paying government jobs. But the news was better for the Washington region: Loudoun County in the D.C. suburbs again ranked as the highest-income county in the nation. Three other Washington-area counties also made the nation's Top 10 list: Howard County in the No. 2 spot, Fairfax County in third place and Arlington County in sixth.

The Washington Post analyzed data from the American Community Survey, released to the public on Thursday, to examine the change in median household income from 2015 to 2016 across the 68 counties that the Census Bureau defines as "biggest city core" counties.

Only seven of the 68 counties saw declines in median income. The largest drop was in the wealthy D.C. suburb of Alexandria , where median household income declined more than 3 percent, from $90,848 to $87,920. The others included the District and five areas with lower incomes than the District: Harris County, Tex., which includes Houston; New Orleans's parish of Orleans; Hartford County, Conn.; Philadelphia County, Pa.; and Monroe County, N.Y., which includes Rochester.

None of the declines was large enough to be statistically significant, but all of these areas missed out on the statistically significant gains that 27 counties recorded, in contrast.

Twelve of the 68 urban-core counties, including the Bronx; Mecklenburg and Wake counties in North Carolina; and Suffolk County, which includes Boston, saw median incomes rise at least 6 percent.

The largest increase in median household income was in San Francisco, where median incomes rose 11.7 percent to reach $103,801. The city of Baltimore saw a 5.4 percent increase to a median income of $47,350. Median income in Wayne County, Mich., which includes Detroit, grew 3.1 percent, to $43,464.

D.C. Council member David Grosso (I-At Large) said he was surprised to hear that the District's median household income was not rising after several boom years.

"It seems like a fairly small number to me right off the bat. At the same time, it's something we need to keep an eye on," he said.

Spokesmen for the chief financial officer in the District and for the city of Alexandria cast doubt on the significance of the census findings. Alexandria's Craig Fifer raised questions about the reliability of the data, including whether the sample size was large enough in a small city. A spokeswoman for D.C. Mayor Muriel E. Bowser (D) did not respond to a request for comment.

But for Stephen Fuller — who runs the Stephen S. Fuller Institute for Research on the Washington Region's Economic Future at George Mason University — the sluggish growth didn't come as a surprise.

"It's slow slippage. We are a wealthy metro area, so we don't notice around the edges that we're actually getting a little tattered," he said. "The D.C. government needs to recognize the problem. . . . A lot of people are in denial because we look rich. See all the cranes on the skyline? How could we not be doing well?"

Those cranes mask the reality that the District has fewer high-paying government jobs and more moderate-income hospitality, restaurant and education jobs than it did just a few years ago.

"We've specialized in generating more moderate-wage jobs," he said. "It catches up to you. You can only grow slower than the other people for so long before you drop behind them. You've lost your advantage."

Grosso said he thinks the District's demographics are at the root of the census numbers: Young people with just-out-of-college salaries flock to the city, but as they start families and their incomes grow, the high-earning couples with children tend to leave for more affordable D.C. suburbs or other regions of the country. He hopes the District's new paid family leave policy and improving public schools can persuade parents to stay put.

"It is an indicator that we need to keep an eye on this, encourage people to live and make their families and make their homes here in the District of Columbia," he said.

Kyley McGeeney, a former resident of the District, left when she wanted to raise children, just as Grosso said is common. She came to the District at 23 to work as a statistician and eagerly moved into an apartment on nightlife-filled U Street. Then she met someone in the District and married him. At 26, hoping to have children and seeking a more spacious home to raise them in, McGeeney and her new husband bought a house in Silver Spring.

"I used to joke, 'We can't have a stroller on U Street,' " she said.

She remembers renting a Zipcar to go take a look at the suburbs and driving into the neighborhood where she now lives. "There were five birthday parties. Two block parties. Moon bounces! People were walking around drinking wine. I was like, 'Is this real life?' "

Now the McGeeneys have a 2-year-old and a 4-year-old, and they're building an addition on the house.

Zach Lipson, a real estate agent, said he works with families like the McGeeneys frequently. "A lot of them are moving out of the city as they get into their early to mid-30s," he said.

Lipson, 34, knows the pattern personally. After 10 years of living in the District, he moved to Bethesda with his wife last year because they wanted to have a baby. They now have an 11-week-old daughter.

It's mostly the high price of city housing, the lack of space and the schools, Lipson said. But there's another factor unique to the District that he says may play a small part in motivating middle-class D.C. renters to become suburban buyers: They don't want to settle down permanently as homeowners in a place where they don't have congressional representation.

"In the back of your mind, there's some sort of impact," he said. "At the end of the day in D.C., where do your tax dollars go? Kind of at the federal government's will."

William Frey, a demographer in the Metropolitan Policy Program at the Brookings Institution, said the region is full of families like the McGeeneys and Lipsons.

"D.C. is probably most distinctive of any city in the country because of the young people that come here. I always like to say that D.C. rents people; they don't own them," he said. His theory is that the poor job market following the 2009 recession caused those young people to stay in the District longer than they otherwise would have and, in some cases, delay starting families. Now that jobs are back in many other parts of the country, many of those residents are leaving the District and moving to the suburbs or elsewhere.

"The District held on to a lot more of those people who couldn't get mortgages or maybe put off having families," he said. "Everything about this is unique to D.C."

Demographers and politicians alike frequently debate which peers the District can fairly be compared to — it is not a state but more of an independent entity than any other city in the nation. Grosso said he thought the urban-core counties were a good choice for comparing household incomes. Frey suggested the 50 largest cities in the nation would be a better basis of comparison and said that on that list, too, the District fell behind slightly while almost all other cities gained: Just six cities out of 50 saw their median household income fall in this year's American Community Survey.

Frey pointed out that the changing fortunes of D.C. residents did not affect everyone equally. The median household income of white residents, who make up 36 percent of the District, rose $2,568, to $127,369, while the median income of black residents, who make up 46 percent, fell $3,631, to $37,891.

This drastic inequality was mirrored in some but not all of the largest cities in the nation: The median black family makes $104,722 less than the median white family in San Francisco but $16,809 less in Jacksonville, Fla.