Childbearing among unmarried women surged to its highest level on record, health officials said. Across-the-board increases in birth rates for women ages 15 to 44 drove the total U.S. fertility rate—the estimated average number of births for women in their lifetimes—in 2006 to its highest mark since 1971, the U.S. Centers for Disease Control and Prevention said in a report. Births to unmarried girls and women reached their highest levels in 2006 since the government began tracking such statistics in 1940.

Unmarried girls and women accounted for 38.5 percent of all U.S. births last year, up from 36.9 percent in 2005. Among blacks, they accounted for 70.7 percent of births. Among Hispanics, it was 49.9 percent and among whites 26.6 percent. The out-of-wedlock birth rate rose by a sharp 7 percent last year.

From about 1995 to 2002, statistics on births by unmarried women remained pretty stable, but since then they have been on the way up. About 40 percent of unmarried women giving birth are in cohabiting relationships. “There certainly is greater acceptance of children being born out of wedlock. For instance, you see many fewer marriages after conception but before birth than you did in the past,” said David Landry of the Guttmacher Institute in New York.

The U.S. fertility rate was about 2.1 births per woman of child-bearing age over her lifetime. This is the first time since the early 1970s that the rate was above the replacement level, at which a given generation can replace itself, the CDC said.