LORAIN, Ohio — It used to be called illegitimacy. Now it is the new normal. After steadily rising for five decades, the share of children born to unmarried women has crossed a threshold: more than half of births to American women under 30 occur outside marriage.

Once largely limited to poor women and minorities, motherhood without marriage has settled deeply into middle America. The fastest growth in the last two decades has occurred among white women in their 20s who have some college education but no four-year degree, according to Child Trends, a Washington research group that analyzed government data.

Among mothers of all ages, a majority — 59 percent in 2009 — are married when they have children. But the surge of births outside marriage among younger women — nearly two-thirds of children in the United States are born to mothers under 30 — is both a symbol of the transforming family and a hint of coming generational change.

One group still largely resists the trend: college graduates, who overwhelmingly marry before having children. That is turning family structure into a new class divide, with the economic and social rewards of marriage increasingly reserved for people with the most education.