Nearly 543,000 babies were born too soon in 2006 found a report by the National Center for Health Statistics. The nation's preterm birth rate -- birth before 37 completed weeks gestation -- rose to 12.8 percent in 2006, or a 36 percent increase since the early 1980s.

The federal government report attributed much of the increase to the growing number of late preterm infants -- those born at 34 to 36 weeks gestation -- which increased 25 percent since 1990.

The report also noted an increase in preterm births to Hispanic women, while rates were unchanged for non-Hispanic whites and blacks. However, black women continue to have the highest preterm birth rate, at 18.5 percent.

Jennifer L. Howse, president of the March of Dimes, said the preterm birth rate continued to rise despite the fact that multiple births, a known risk factor for preterm birth, have begun to stabilize. The rate of twin births was unchanged in 2005 and 2006, and triplets and higher order multiples declined 5 percent in 2006.