Nearly a third (28 per cent) of all American women with two or more children have children by more than one man, according to a new study.

The report, which is the first nationwide study multiple partner fertility shows that 28 percent of all U.S. women with two or more children looked at data on nearly 4,000 U.S. women who were interviewed more than 20 times over a period of 27 years, as part of the National Longitudinal Survey of Youth.

Dr Cassandra Dorius, who carried out that research, said she was surprised at how many women had children by more than one father.

Previous studies into the phenomenon examined how common multiple partner fertility is among younger women, or among women who live in urban areas.

But this latest research is the first to assess prevalence among a national sample of U.S. women who have completed their child-bearing years.

The study found that having children by different fathers was more common among minority women, with 59 percent of African American mothers, 35 percent of Hispanic mothers and 22 percent of white mothers with two or more children reporting multiple partner fertility.

Women who were not living with a man when they gave birth and those with low income and less education were also more likely to have children by different men.

Surprisingly, however, the study also found that multiple partner fertility was relatively common at all levels of income and education and is frequently tied to marriage and divorce rather than just single parenthood.

“I was a year into this project before I realized that my mother was one of these women,” Dr Dorius said. “We tend to think of women with multiple partner fertility as being only poor single women with little education and money, but in fact at some point, most were married, and working, and going to school, and doing all the things you’re supposed to do to live the American Dream .”

Family researchers first began studying multiple partner fertility by exploring how men ‘swap families’ after having a child with a new partner, or reduce their financial support and physical involvement with non-residential biological children when their ex-partners live with, or marry, someone new.

Dr Dorius, a demographer at University of Michigan Institute for Social Research said that the raising of children who have different fathers was “a major factor in the intergenerational transmission of disadvantage”.

She said:  “Juggling all the different needs and demands of fathers in at least two households, four or more pairs of grandparents, and two or more children creates a huge set of chronic stressors that families have to deal with for decades.”