New US Census figures show that over a quarter of unmarried women who gave birth in the year ended June 30,  2008 were living with a partner.

It is the first time that the Census Bureau has measured the percentage of unmarried mothers who were not living alone. In Ireland in the first quarter of this year, half of all single women who gave birth – 53 percent – were living with a partner.

“Everybody tends to think of single mothers as being alone with their child, and we wanted to look at whether that was true,” said Jane Dye, the demographer who wrote the report, Fertility of American Women: 2008.

“We found that 28 percent of these women were living with an unmarried partner, whether opposite sex or same sex.” While recent years have seen a huge increase in the numbers of people cohabiting in the US, the statistical category of “single mother” has often blurred the difference between those living alone and those living with a partner.

But recently, the Census Bureau’s Current Population Survey, one of the sources for the fertility report, added a question on cohabitation to make it possible to measure how many new mothers were actually on their own.

According to a study by the US National Center for Health Statistics released in February, about half of cohabiting couples marry within three years, and about two-thirds within five.

However, British figures suggest that cohabitation is highly unstable, leaving children in a very vulnerable position.

Data from the UK’s Millennium Cohort Study showing that the children born to cohabiting parents were only half as likely to live with both parents throughout childhood compared with children born to married parents.

Furthermore, Australian data suggests that cohabitation prior to marriage is one of a number of factors which can lead to marital breakdown. Researchers from the Australian National University, found that the bulk of research on cohabitation and marital instability finds that cohabitation before marriage is linked to a greater probability that the marriage will fail.

According to the study, titled “What’s Love Got to do With It?”, 20 per cent of couples who had children before marriage, either from a previous relationship or the same relationship, were separated compared to just nine per cent of couples without children born before marriage.

Andrew Cherlin, a demographer at Johns Hopkins University, said he was surprised that the number of mothers living with a partner was not higher, since previous estimates had put it at around half of unmarried mothers.

“Cohabitation until recently was invisible in government reports,” Mr. Cherlin said. “It’s data we need. If we’re concerned about stable environments for children, we have to know whether we should be focusing our efforts on helping cohabiting couples keep their relationship together, or whether we’re talking about unmarried teen mothers who are on their own.”

According to the Census Bureau report, released Thursday, unmarried women made up 1.5 million of the 4 million women ages 15 to 44 who gave birth between June 2007 and June 2008.

The report also found that the proportion of mothers of newborns who were in the labor force had increased to 61 percent in 2008, from 57 percent in 2006. Similar studies have shown that the percentage of working mothers with newborns rose to a peak of 59 percent in 1998 and then declined, but that it has lately been rising close to peak levels.

The report also looked at the effects of women’s increasing educational attainment on their childbearing. Women who continued their education into their 20s experienced lower fertility levels at younger ages but higher fertility at older ages, once they completed their education.

According to the fertility report, which is published every two years, 18 percent of all women ages 40 to 44 in 2008 were childless, down from 20 percent in 2006 but still far higher than the 10 percent in that age group who were childless in 1976.

By the time women reached their 40s, the report found, they had averaged 1.9 births, a substantial decline from the 3.1 births such women averaged in 1976, when the Census Bureau began collecting fertility data.

Nationally, one in four mothers who recently gave birth lived in poverty in 2008. About 20 percent of the women who gave birth during the year were foreign-born, the report found.