Steven Martin, a sociologist at the University of Maryland, said the paper for the Council on Contemporary Families said that the rate of births to U.S. women averages out to a level at which the population will reproduce itself by births alone, without counting any population growth fueled by immigrants.

The National Center for Health Statistics found the number of U.S. births increased by 3 percent in 2006 -- reaching a level not seen since the baby boom in 1961.

Affluent families are spearheading a return to larger, more traditional families of three to four children but Martin said that trend is confined to only a tiny sliver of the population, not enough to affect the overall fertility rates and is not typical of what is happening for the other 98 percent of the population.

The increase in fertility from 2005 to 2006 is concentrated among non-marital births -- 117,000 babies born to mothers, mostly in their 20s and 30s, who aren't married.