Last year, the national premature birth rate reached the lowest it's been in a decade.
Still, the March of Dimes has given the U.S. a "C" grade in its annual premature birth report card.
In 2011, the pre-term birth rate was 11.7 percent. That's down from a high of 12.8 percent in 2006.
Just four states earned an "A" for their preemie prevention efforts -- Maine, New Hampshire, Oregon and Vermont.
Forty-five states and Washington D.C. improved their pre-term birth rates last year.