Over the course of a year, Illinois’ population dwindled by 33,703 people, dropping it from fifth place to sixth place on the list of most populous states in the country.

That decline is one of the worst, according to data released Wednesday by the Census Bureau. Many neighboring states, like Indiana, Iowa, and Michigan, all saw moderate population gains during the same period. Pennsylvania is now ranked fifth.

Brian Harger, a research associate at Northern Illinois University’s Center for Governmental Studies, said a decline means the state has the potential to lose seats in the U.S. House.

As for why the decline is happening, Harger said it could be linked to a declining birthrate for the state, a drop in domestic migration — meaning people coming to Illinois from other states — and a “leveling off” of foreign migration.

“Rural counties and downstate Illinois have taken the brunt of population declines in the past, but the number of people in Chicago, which has propped up the state, has leveled off,” Harger said. “The decline could mean fewer federal dollars, and if it continues it won’t bode well for economic development.”

The center, which is also a State Data Center for the U.S. Census Bureau, says the state has lost over 88,000 people since 2013, in part due to less migration and birth rates. Economic recovery has also “lagged,” Harger said, and that isn’t attractive to residents.

In a statement, Liz Schuh, principal policy analyst for the Chicago Metropolitan Agency for planning, said that while the narrow window of the study can be misleading, “Census data over several years does indicate stagnant or even declining population for our region and state.”

“Among some positive signs — for example, increased overall diversity, despite losses in some races, ethnicities, and geographic areas — the ability to retain current residents poses near- and long-term economic and social challenges for metropolitan Chicago and Illinois,” Schuh said.

If trends continue, the state could find itself in a downward spiral, Harger says, though he doesn’t believe Illinois has reached that point yet.

“A lot of states have witnessed similar issues and come back from them,” Harger said, referring to Pennsylvania, which he says has seen a population increase of about 103,000 since 2010. “What we’re seeing is the state aging with the rest of the country and industrial world, as well as changes in the birthrate and a young workforce. Those are issues all states will have to deal with.”