For Utah, leading this year may simply be a result of not having yet been hit by the very factors slowing growth nationally.

“We’re lagging behind those trends in fertility and household formation and all,” said Pamela Perlich, director of demographic research at the University of Utah’s Kem C. Gardner Policy Institute. They may be slow in coming, but those same drags on growth may well hit Utah, too, she said.

A state’s fortunes can quickly reverse, as they did for North Dakota this year. After four years as the nation’s fastest-growing state, North Dakota dropped out of the Top 10 altogether, thanks to residents fleeing for other parts of the country.

Only eight states posted declines, with West Virginia shrinking the most, relatively, by 0.5 percent. Illinois lost more residents than any other state, shedding nearly 38,000, driven by people moving out.

The West dominated the top of the list, accounting for seven of the 10 fastest-expanding states. That region and the more-populous South each had population growth of nearly 1.1 percent over the year.

“The movement to the South and West is a very long-term trend,” said Mr. Passel of Pew, adding that those regions attract older residents of the Northeast and Midwest looking for more temperate places to retire.

The Midwest expanded by nearly 0.2 percent, while the population in the Northeast remained virtually unchanged. Both regions lost more residents than they gained from migration, though that was offset by more births than deaths.