the vast majority , 71 percent , had at least some college education before giving birth .
by contrast , births to women younger than 20 declined from 533,000 in 1990 , except for a spike in 2006 and 2007 .
most mothers of newborns ( 54 percent ) had at least some college education in 2008 , an increase from 41 percent in 1990 , one in 11 had a mother in that age group .
the average age trend was reversed in 1990 , according to the study , " the new demography of american motherhood . "
one in seven babies -- or 14 percent of a total of about 4 million births -- were born to older mothers in 2008 .
while most women giving birth are doing it within the context of marriage , researchers said a record 41 percent of births were to unmarried women in 2008 .
among mothers 35 or older , 71 percent had more children are born to women older than 35 than to teenagers , a change born of medical science , later marriages and evolving attitudes about motherhood , according to a new study released thursday .
the trend toward mothers who are older than 35 years old .
but that does n't mean women are waiting for the right moment : the study also found that half of mothers surveyed said parenthood " just happened . "