For the sixth year in a row, the rate of premature births has gone down in the U.S., but the country still lags behind every other developed nation in that category, CBS News reported.

The new data comes from a report from the March of Dimes, a nonprofit pregnancy organization. In 2012, 11.5 percent of babies were born too soon, down from last year and a 15-year low. Still, March of Dimes gave the U.S. a "C" and said the nation is lagging behind other industrialized countries.

Babies born prematurely are defined as being birthed at least three weeks before their due date and about 450,000 babies meet this criteria every year. On average that figure is 1 in every 9 babies.

"Although we have made great progress in reducing our nation's preterm birth rate from historic highs, the US still has the highest rate of preterm birth of any industrialized country. We must continue to invest in preterm birth prevention because every baby deserves a healthy start in life," March of Dimes president Dr. Jennifer L. Howse said in a press release. "A premature birth costs businesses about 12 times as much as uncomplicated healthy birth. As a result, premature birth is a major driver of health insurance costs not only for employers."

Premature births are not always forced upon a mother and sometimes are elected. That practice has become strongly discouraged by several experts in hospital systems, USA Today reported.

Premature births are the leading cause in newborn deaths and also lead to lifelong health and developmental problems. Premature births can also be prevented by not smoking during pregnancy, not drinking and eating healthy foods.

"It's really embarrassing to say that we don't even know why women go into labor at full term," Craig Rubens, executive director of the Global Alliance to Prevent Prematurity and Stillbirth, based at Seattle Children's Hospital, "much less why some go into labor early."