Since the early 1990s, the share of out-of-wedlock, cohabiting births has grown from 11 percent to 24 percent, while those to noncohabiting, single mothers has remained steady at 16 percent.

Sometimes referred to as the “poor person’s marriage,” cohabitation is growing fastest among high school graduates with children. Between the 1997-2001 and 2002-2009 periods, it grew from 23 percent to 32 percent, according to Sheela Kennedy, a researcher at the University of Minnesota. For mothers with some college attendance, it grew from 15 percent to 23 percent during that period. Among those with four-year college degrees, the share has changed from 3 percent to 5 percent.

Daniel Lichter, a Cornell sociologist and past president of the Population Association of America, said the government needs to do more to reflect increasing cohabitation in statistics. Cohabitation status is not on birth certificates, and that can skew policy debates over the government safety net for poor households. It also means a growing trend of fragile families in which cohabitating parents may be more likely to break up can be neglected, he said.