Gates, who specializes in tracking gay populations, said the United States has an estimated 600,000 gay couples and 60 million heterosexual couples.

Maryland did not have a large influx of residents between 2000 and 2010, said Gates, who concluded that the change noted in census data was not due to large numbers of gay households moving here.

Maryland's numbers, he said, represent changing attitudes among gay couples — an increased willingness to stand up and be counted. This increase in self-reporting can be seen in the data, he said. Metropolitan areas that were more accepting of gays in 2000 — Baltimore and the Washington suburbs, for example — showed smaller increases than rural areas such as Garrett and Washington counties.

"It's indicative of the fact that those areas had bigger closets 10 years ago," Gates said.

Kelly and Heether hope that the growing number of gay couples who publicly acknowledge the relationships will bolster the perception that their family, and other gay households, are just like households headed by heterosexuals.

The women live in Timonium with their 5-year-old twins, Katie and Jackson, and run a female-focused sporting goods business called Sports Her Way.

"We're the same families," Heether said. "We're always driving our children to this and to that and asking each other 'Do you have the car seats?' and 'Do we need anything at the store?' and 'Better get milk, we always need milk.'"

A little more than 4,500 of Maryland households with gay partners, about 28 percent, are raising children. About a third of lesbian households and a little more than one-fifth of households with gay male partners have children. For this data, the census includes all youths other than foster children.

Less than 40 percent of Maryland households with heterosexual partners are raising children.

The census does not maintain comparable data from 2000 or 1990 about children being raised by same-sex couples.

Although Kelly and Heether are legal parents of their twins, they have stored photocopies of the children's important legal documents in their cars, offices and home in case there's ever an emergency and Heether is not present.

"We have a fear of going to the hospital and Catherine not being recognized as their mother," said Heether, the twins' biological mother.

Although the women wear wedding rings, they don't see themselves as married. But marriage, they agreed, would help to strengthen their children's legal protections.

"If it would legally help us, streamline all of that and make things simpler … " Heether said, her sentence finished by Kelly: "If it would make our children's protections tighter and better, then great."

Patrick Wojahn, board chairman of the gay-rights group Equality Maryland, hopes that the census numbers will show Maryland residents that there are thousands of children living with same-sex parents who need equal protection under the law.

The gay-marriage issue is likely to return during the 2012 General Assembly, and Kelly and Heether will be there — in a sense. Their family portrait will be displayed in Annapolis during the legislative session as part of an exhibition showing same-sex households.

David Buscher, 39, and Rob Hartmann, 45, a Baltimore couple who have been together for about a decade, also sat for a portrait in artist Katherine Meredith's exhibition bound for Annapolis. The men, who are part-owners of Charmington's cafe, had a wedding ceremony several years ago that is not legally recognized by the state.

They consider themselves married, but felt obliged, Buscher said, to list themselves as "unmarried partners" on the census form. He did not check the "Husband/wife" box when describing Hartmann, he said.

"I think I probably selected 'unmarried partner,' as I would have been thinking about legalities," Buscher said.

The census data released today did not differentiate between same-sex couples who said they were married and those who said they were unmarried partners. Instead, the data grouped the two categories together; the complete breakdown will be released in November.

Eugene Tauber of Tribune Newspapers contributed to this article.