(USA TODAY) -- As more unmarried couples in the USA move in together, more also are getting pregnant, a new report suggests.
The number of births overall to cohabiting women increased from 14% of all births in 2002 to 23% in 2006-10, according to the first federal report on intended and unintended births since 1990.
Data released today by the National Center for Health Statistics show that more than three-quarters of all births to married women were intended, compared with about half of births to cohabiting women and a third of births to women who are unmarried and not cohabiting. Unintended pregnancies include those that are both mistimed and unwanted, according to the definition in the National Survey of Family Growth.
The data are based on in-person interviews in either English or Spanish among 12,279 women ages 15-44 between 2006 and 2010. Pregnancies ending in miscarriage, stillbirth or abortion were not counted.
"Because there's an underlying shift in the population that more people are cohabiting, that leads to more unintended pregnancies and unintended births," says Larry Finer, director of domestic research at the non-profit Guttmacher Institute in New York. Finer has seen the new report and used similar data for a report last year.
Researchers thought unintended births overall might drop with the introduction of longer-acting contraceptive methods, says statistician William Mosher, the report's lead author. But they're not used by enough people to make a difference. "If they are used by larger proportions of people, we might see a decrease," he says.
The report found that about 37% of births in the USA were unintended at the time of conception, a percentage that hasn't changed since 1982. The proportion of unintended births declined among married white women, but those accounted for a smaller proportion of births. Meanwhile, births to single or cohabiting women rose.
The report also reveals more details about contraception. In 2008, for example, 19% of births were unintended; 36% of women who had an unintended birth said they didn't use contraception because they thought they couldn't get pregnant. But 23% of those women said they "didn't really mind if I got pregnant."
"A lot seems to have to do with the fact people are increasingly ambivalent about whether or not to have a child," says Karen Guzzo, a sociologist at Bowling Green (Ohio) State University. "They're in this committed relationship and are often cohabiting and not trying hard to avoid having a child, but they're not trying to have one, either."