“The surprising thing is that we’ve had this really big upswing in births to unmarried mothers in such a short period of time,” said Stephanie Ventura, the author of the report commissioned by the CDC’s National Center of Health Statistics.

In the U.S., nearly 40 percent of births were to unmarried women in 2007, compared with about 34 percent in 2002, according to data first released in March but now being compared with that of other countries.

The report shows that teenagers accounted for only 23 percent of 2007’s non-marital births, down from 50 percent in 1970, demonstrating that birth rates for unmarried women in their twenties and over have risen considerably over the period.