About 18.1 percent of all single women who became pregnant opted to move in with their boyfriends before the child was born, according to 2006-2010 data from the government's National Survey of Family Growth, the latest available. That is compared to 5.3 percent who chose a post-conception marriage, according to calculations by Daniel Lichter, a Cornell sociologist.
As recently as the early 1990s, 25 percent of such couples got married.
Cohabiting mothers are spurring increases in out-of-wedlock births, now at a high of 41 percent. In all, about 60 percent of all births during the 2000s were to married mothers, compared to 24 percent to cohabiting mothers and 16 percent to non-cohabiting mothers. That was the first time that cohabiting births exceeded births from single mothers who weren't living with their child's father.
Since the early 1990s, the share of out-of-wedlock, cohabiting births has grown from 11 percent to 24 percent, while those to noncohabiting, single mothers has remained steady at 16 percent.
Sometimes referred to as the "poor person's marriage," cohabitation is growing fastest among high school graduates with children. Between the 1997-2001 and 2002-2009 periods, it grew from 23 percent to 32 percent, according to Sheela Kennedy, a researcher at the University of Minnesota. For mothers with some college attendance, it grew from 15 percent to 23 percent over that period. Among those with four-year college degrees, the share has changed little, from 3 percent to 5 percent.
Lichter, a past president of the Population Association of America, said the government needs to do more to reflect increasing cohabitation in statistics. Cohabitation status is not included on birth certificates, and that can skew policy debates over the government safety net for poor households. It also means a growing trend of fragile families in which cohabitating parents may be more likely to break up can be neglected, he said.
Researchers at Harvard and Cornell universities have found that only about half of mothers who were cohabiting when their child was born were still in relationships with the biological father five years later. "The latest results seem to indicate that marriage, as a context for childbearing and childrearing, is increasingly reserved for America's middle- and upper-class populations," Lichter said.