The number of mixed-race babies has soared over the past decade, new census data show, a result of more interracial couples and a cultural shift in how many parents identify their children in a multiracial society.

More than 7 percent of the 3.5 million children born in the year before the 2010 Census were of two or more races, up from barely 5 percent a decade earlier. The number of children born to black and white couples and to Asian and white couples almost doubled.

“I think people are more comfortable in identifying themselves, and their children, as mixed race,” said William H. Frey, a Brookings Institution demographer who analyzed detailed census data on mixed-race infants. “It’s much more socially acceptable, more mainstream, to say, ‘That’s what we want to identify them as.’ ”

The District, Maryland and Virginia all lag behind the national average in multiracial children, but that is changing rapidly.

In both Fairfax County and Montgomery County public schools, for example, about 4.5 percent of the student body is more than one race. With 8,200 in Fairfax schools, the ranks have multiplied six times in the 15 years since the district started keeping count. The 6,500 mixed-race students in Montgomery would fill three high schools.

Though mixed-race children are increasingly common, parents say it can be tricky talking to them about race.

Thien-Kim Lam, a Silver Spring mother of two, is a first-generation Vietnamese American married to an African American. Their daughter, who is 6, has started to notice that she doesn’t look like her mother.

“At the very beginning, I thought if I didn’t talk about race, she’d just be colorblind,” said Lam, who writes a blog on raising biracial kids in a race-conscious world; its name comes from what she tells strangers who see her for the first time with her children — “I’m Not the Nanny.”

“But it’s important to teach them to be proud of who they are,” she added. “I see it as a chance to teach my daughter to accept her two parts, a new combination — half me and half of her dad, the perfect color in between.”

Frey said the census statistics on children with black and white parents in particular show a country that is advancing toward the day when race loses its power to be a hot-button issue.

People who identify themselves as one race tend to be older. They reflect a society in which laws prohibited interracial marriage and states such as Virginia enforced a “one drop” rule designating anyone as black if they could trace even one drop of their blood to an African American ancestor. President Obama, for example, identified himself as one race — black — on his census form, even though his mother was white.

When Lisa Rosenberg was growing up as a biracial kid in a racially diverse neighborhood in New York during the 1960s and 1970s, she had many mixed-race friends. But her late father, who was African American, counseled her to always say she was black, even though her mother is Jewish.

“I think to protect me, he’d say, ‘To the world, you’re black,’ ” said Rosenberg, a therapist who often speaks at schools on addressing the issue of race to children.

But Rosenberg, who lives in Montclair, N.J., checked more than one box for race on her census form, as she did for her two children. She expects more pointed questions from her daughter when she enters middle school.

Young people, typically younger than 15, are much more likely to be identified as mixed race. Among infants younger than 1, there are 17 mixed-race children for every 100 infants whose parents said they are black alone. A decade ago, there were nine.

“One out of six kids who used to be thought of as just black will now grow up thinking of themselves as white and black,” Frey said. “This is a huge leap. This is a ray of hope that we’re finally moving into an era where this very sharp black-white divide is breaking apart.”

Lam said she has already fielded questions from her 6-year-old.

“Her classmates have asked why she and I don’t have the same skin color,” she said. “We talk about race with our children when things come up, like if a stranger says something. Ultimately, we’re all concerned about raising our children to be good people.”