A growing share of unmarried middle-aged women is having kids.

New data from the Centers for Disease Control and Prevention (CDC) show that birth rates among unmarried women in the U.S. between the ages of 35 and 39 shot up 48 percent between 2002 and 2012. For older women between 40 and 44 years old, this rate—while very low—rose 29 percent between 2007 and 2012.

That means for every 1,000 women 35 to 39 years old who are not married, there were 31 births in 2012, compared with 29 births in 2007 and 21 births in 2002. For unmarried women 40 to 44, this figure edged up from seven births in 2007 to nine births in 2012.

U.S. women are much more likely to have children out of wedlock than they used to. Roughly four in 10 U.S. births are to unmarried women, a figure that peaked in 2009 and hasn’t budged much since, according to the CDC.

The rise of the unmarried mom in America has stirred concern for years. Married parents tend to be more stable and have higher household incomes, which benefit children. And a gap is growing between America’s married parents, who tend to be better-educated and higher-earning, and unmarried parents, who are likelier to be less-educated and lower-earning. This so-called marriage divide could fuel social and economic inequality.

But the CDC’s latest findings show that trends related to unmarried moms vary greatly by age: It’s becoming much more common for middle-aged, unmarried women to have kids, as taboos ease. At the same time, younger women are delaying having children, to pursue career opportunities. Unwanted teen births, meanwhile, have plummeted, especially among black and Hispanic women.

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