“It changes the dynamic of the household,” she says. “I had a friend who put off her marriage. Got pregnant, and she’s like, ‘let’s just wait, ’cause we don’t know if we’re going to be able to make it through this.’ ”

That attitude reflects a sea change in family life: For the generation under age 35, nearly half of all births are now outside marriage. This family structure, once common mainly among African-Americans and the poor, is spreading across races and into the middle class.

Factor in education, though, and the difference is stark, raising concerns of a new class divide. Among young women without a college degree — those like Michelle Sheridan — 55 percent of births are outside marriage, according to an analysis by the research group Child Trends. For those with at least a four-year degree, it’s just 9 percent.