Nearly half of women ages 15 to 44 say their "first union" was cohabitation rather than marriage, according to a new survey by the Centers for Disease Control and Prevention, USA Today reports. As a first union, 48 percent of women moved in with their male partner, up markedly from 43 percent in 2002 and 34 percent in 1995. Just 23 percent of first unions were marriages, down from 30 percent in 2002 and 39 percent in 1995. "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." Experts say the numbers show living together is increasingly being used as a "testing ground" for marriage. Within three years of cohabiting, 40 percent of women had transitioned to marriage, 32 percent remained living together and 27 percent had broken up. The median duration of first cohabitation is 22 months, up from 20 months in 2002 and 13 months in 1995. The report also found that 19 percent of women became pregnant and gave birth in the first year of a first premarital cohabitation.