Data on unmarried-couple households has been tabulated and growing since 1996, according to a report on "Improvements to Demographic Household Data in the Current Population Survey: 2007," by Rose M. Kreider of the Census Bureau's housing and household economic statistics division.

In 1996, there were 2.9 million cohabitating households, the report said. By 2006, that number had grown to 5 million cohabitating couples, comprising 4.4 percent of the nation's households. And the number of those households with children is growing. In 1996, an estimated 57 percent of the households had children. That had grown to an estimated 61 percent in 2006.

"This is a concern to policy makers since research shows that children living with married parents fare better on average than those with cohabitating parents," the report said. "The ability to track children living with two unmarried parents is important to building an understanding of how their characteristics might differ from families with two married parents."

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