National Center for Health Statistics

Unmarried couples who live together are staying together longer than in the past — and more of them are having children, according to new federal data that details just how cohabitation is transforming families across the U.S.

For almost half of women ages 15-44, their “first union” was cohabitation rather than marriage, says the report from the National Center for Health Statistics. For less than one-quarter, the first union was marriage. The report was based on in-person interviews conducted between 2006 and 2010 with 12,279 women ages 15-44.

“Instead of marriage, people are moving into cohabitation as a first union,” said demographer Casey Copen, the report’s lead author. “It’s kind of a ubiquitous phenomenon now.”