In 1976, census data show, 59 percent of women ages 40 to 44 had three or more children, 20 percent had five or more and 6 percent had seven or more.

By 2006, four decades after the Supreme Court declared a constitutional right to use birth control (and the last year available from census studies), 28 percent of women ages 40 to 44 had three or more children, 4 percent had five or more and just 0.5 percent had seven or more.

"Three is still O.K.," said Michelle Lehmann, the founder of lotsofkids.com and a mother of eight children who lives outside Chicago. "When you have four, people start raising eyebrows. When you go to five, people are like, 'No way.'"