The rise in the number children born outside marriage in the US is being now being driven by a big increase in the number of children who are born to cohabiting couples, according to a new study.

More than one-half of non-marital births occur within cohabiting relationships. In 2001, 52 percent of all nonmarital births took place within a cohabiting union, compared with 38 percent in the early 1990s. 

It also found that a majority of all births that occurred outside of marriage were unintended– either mistimed or not wanted. Fifty percent of births to cohabiting couples were unintended. Sixty-five percent of birth to lone mothers were unintended.

The research, carried out by the Child Trends think-tank, found that between 1970 and 2009, the percentage of all births that took place outside of marriage increased from 11 to 41 percent.

The study found that, while the rise affected women from all social groups, the increases in nonmarital births have been more dramatic among white and Hispanic women than among black women. 

The report, Childbearing Outside of Marriage: Estimates and Trends in the United States, showed that women in their twenties had the highest levels of nonmarital childbearing. 

In 2009, 62 percent of all nonmarital births occurred to women aged 20-29. 

Teenagers account for a diminishing share of all births outside of marriage. In 1970, teenagers accounted for almost half of all nonmarital births, by 2009 this had fallen to one in five.

According to the report, less than one-half of all nonmarital births are first births.

The report also notes research suggesting that marriage would bring some economic advantages to unmarried women and their children, particularly for those from the most disadvantaged backgrounds.