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 American 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 a recent peak in 2007.

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, Mr. 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 North east and Midwest that were losing residents to migration have resumed losing them.