Health officials say nearly 1 in 4 babies are born to unmarried couples who are living together, a significant jump from a decade ago.
The government previously said more than 40 percent of births are to unwed mothers. But a report released Thursday offers new detail showing most unwed births occur in cohabiting couples.
The Centers for Disease Control and Prevention study was based on face-to-face interviews with more than 22,000 men and women during 2006 through 2010.
About 23 percent of the reported births were to unmarried heterosexual couples who were living together when the child was born. That was up from 14 percent when a similar study was done in 2002. The CDC did not speculate on reasons for the increase.