Teen pregnancy rates may have reached an all-time low, but the number of kids born to unmarried, cohabitating parents is up.

In fact, according to data from National Center for Health Statistics, more than two in every 10 babies in the US is born to such a couple.

Numbers collected from 2006-2010 show the percentage of first births to women living with a male partner jumped from 12 percent in 2002 to 22 percent during the four-year study period.

“We were a little surprised in such a short time period to see these increases,” says demographer Gladys Martinez, lead author of the report.

Karen Benjamin Guzzo, a sociologist at Bowling Green State University in Bowling Green, Ohio, thinks the recession is partially responsible for the rapidly shifting numbers, saying, “In a perfect world, they would prefer to be married, but where the economy is now, they’re not going to be able to get married, and they don’t want to wait to have kids.”

But maybe the new numbers aren’t as surprising as they seem. Kelly Raley, a professor of sociology at the University of Texas-Austin, says since more people live together before marriage these days, a rise in births in those couples is to be expected.

And there’s another reason she thinks it might be happening: “It just could be that it’s OK now to have a kid outside of marriage.”

[USA Today]