More American couples are now living together — and having children — without marriage, and most cohabiting couples break up without marrying, a new federal study finds.

Fifty-five percent of women have been in an unmarried cohabitation relationship with a man by the time they are 25 years old, according to “First Premarital Cohabitation in the United States: 2006–2010 National Survey of Family Growth” (click here to see the PDF document). The study by researchers Casey E. Copen, Kimberly Daniels and William D. Mosher was published Thursday by the National Center for Health Statistics.

“Instead of marriage, people are moving into cohabitation as a first union,” Copen told USA Today. “It’s kind of a ubiquitous phenomenon now.”

About 1-in-5 women became pregnant within the first year of their first cohabiting relationship, the study found, and most such relationships don’t lead to marriage. Thirty-two percent break up in less than three years while 27 percent continue living together after three years. Only 40 percent of couples marry within three years.

The study found that cohabitation was particularly common among women with less education, and among Hispanics. For women without a high-school diploma, 76 percent had cohabited by age 25, compared to 36 percent of women with a college degree. Sixty-five percent of U.S.-born Hispanic women had cohabited by age 25, compared to 57 percent for white women and 51 percent for black women. Asian-American women were least likely to cohabit (19 percent by age 25), and foreign-born Hispanic women were less likely to cohabit (53 percent by age 25) than Hispanics born in the United States.

The rise in cohabitation is bad news, according to Tony Perkins of the Family Research Council, who cited research showing that unmarried couples have lower incomes and their relationships are less stable.

“Studies from our Marriage and Religious Research Institute (MARRI) show that these adults are less engaged in the economy because they don’t marry,” Perkins wrote in his “Washington Update” report. “Cohabitation also crowds out marriage. When more people live together over time than marry, the effects on the nation and its economy are disastrous, because of all the attendant goods of marriage that are lost.

“These unstable relationships are also incredibly unhealthy for kids,” Perkins continued. “Research is clear: these couples are more likely to be unhappy, cheat, feel depressed, experience abuse, and divorce (if they ever do get married).”