More than six out of 10 women who give birth in their early 20s are unmarried, the U.S. Census Bureau said Wednesday.

Census demographers said that single motherhood, while on a steady uptick since the 1940s, has accelerated in recent years. The birth rate for unmarried women in 2007 was up 80 percent in the almost three decades since 1980, the report said. But in the previous five years alone, between 2002 and 2007, it was up 20 percent.

Echoing the findings of many academic studies, the Census Bureau report said women with college degrees and higher household incomes are far less likely to be single mothers than are women who have lower household incomes and less education.

Overall, 36 percent of all births in the United States were to unmarried mothers in 2011, the year that the census analyzed from answers provided in the American Community Survey.