Hunterdon County, New Jersey, is rich. It ranks sixth nationally among counties in median household income, and has for decades exemplified the American outer-ring suburb.

But in one crucial measure, this exurban enclave 60 miles west of Manhattan resembles old mill communities in northern New England or impoverished regions of Appalachia. The measure is death as compared to birth, otherwise known by demographers as “natural increase.” In this case, it’s negative.

Hunterdon County residents gave birth to 3,590 babies between 2013 and 2016. But even more residents — 3,647 — died. Go back to a four-year period two decades earlier: 5,882 births, 2,947 deaths.

With new county-level census population estimates due out this week, demographers will track not just the population gains and losses but the components of change: births, deaths and the movement of people. And while migration, which counts people moving both domestically and immigration, gets most of the attention, changes in birth and death patterns are increasingly important.

Some U.S. communities that until recently were considered demographic boom towns are now caught up in a downward demographic mix: young people having fewer children, the boomer generation getting older. And migration patterns, stalled by the recession, are resuming, but only in certain parts of the country.

Through 2016, about one in four outer-ring suburbs were experiencing more deaths than births, including 18 of 30 such counties in New York, New Jersey and Pennsylvania.

Some of the once-fastest-growing counties in the United States are growing no more, and nationwide, the birthrate has dropped to levels not seen since the Great Depression.

“It is one of the biggest puzzles of my career as a demographer,” said Kenneth Johnson, a sociologist at the University of New Hampshire who has studied the various components of population change for years. “Each year when new data comes out, I expect to see a significant uptick in births, but I have yet to see it.”

More than 1,200 counties in the United States — home to one in seven Americans — had a negative natural increase in population in 2016. In total, 1,700 counties experienced a negative natural increase at least one year this decade.

For many counties, this makes migration especially important for population stability and growth. Counties in the Northeast and Midwest that have traditionally lost residents to the South and West are having a harder time propping up their population numbers.

Some maintain their numbers because of immigration, but U.S. immigration policy is now a subject of debate, and a smaller number of immigrants would put more pressure on counties facing population loss.

The nation’s sprawling growth pattern has taken its share of criticism; it’s associated with long-distance commuting, environmental degradation and urban decay. But population stagnation in places that had been growing will most likely bring its own sets of problems, including pressures on real estate values and eventual shrinking of political representation.

And it starts with babies. The estimated lifetime births per woman is down 16 percent from its 2007 peak.

New Census Bureau projections say that Americans over 65 will outnumber children under 18 for the first time in U.S. history by 2030, and that “a rising number of deaths will increasingly offset how much births are able to contribute to population growth.”

Migration dropped significantly during the recession, Johnson said, but has returned to pre-recession patterns, albeit at a slower pace. Florida, Texas and Arizona have all seen population inflows resume, for example, while states in the Northeast and Midwest that were losing residents to migration have resumed losing them.

Births, on the other hand, have not returned to pre-recession patterns. They’ve instead skewed sharply in a negative direction.

“The number of deaths continued to rise during the recessionary period, but births have yet to rebound from the impact of the recession,” Johnson said.

This has been most pronounced in places that have long struggled economically, but it’s also now increasingly common in well-off suburbia.

In many areas, young people, in addition to having fewer children, are not as enamored of the suburbs as previous younger generations. This is especially true in cities experiencing urban revivals such as New York.

“Millennials don’t want to be in outer suburbia,” said James W. Hughes, the emeritus dean of the Edward J. Bloustein School of Planning and Public Policy at Rutgers University.

Whereas previous generations of young couples streamed ever farther out to the suburbs in search of larger lots and lower taxes, the current crop prefers Brooklyn, Jersey City and other locales close to the urban core.

Population had grown rapidly for years in the 11 outer ring counties of the New York metropolitan area, Hughes said. But those counties are all like Hunterdon: shrinking, with slowing migration and more deaths than births.

Enrollment at some Hunterdon County school districts is down by as much as 20 percent, he said, and although plenty of older residents remain in place, they will most likely end up paying more to live there as the tax base shrinks along with the population.

The birth-death ratio is declining faster in the suburbs of large cities than in the cities themselves, data compiled by the demographer William Frey of the Brookings Institution shows.

In 2008, there were 1.9 births for every death in what Brookings terms the “emerging” suburbs — places like Hunterdon County where the suburban foothold has not yet overtaken the rural character. Less than 10 years later, the ratio had dropped to 1.5, roughly the rate of rural America last decade.

Not only are many outer suburban counties aging, but they also tend to be disproportionately white, said Frey, which probably contributes to the falling birth-death ratio in suburban counties, since the birthrate is especially low for white women.

In some parts of the South and West, the birth-death imbalance is masked by increasing migration, which still drives population growth. The suburbs there are still popular and attract local, national and international migrants.

Yet many communities in the North are net exporters of migrants — more people leave than move in.

Johnson said the natality data resembles that of another time in history, but without the obvious economic explanation: “The only other time we’ve been in a situation like this has been in the Great Depression itself. There was a drop in women having children, especially young women, and they never made up for never having children young when they got older.”