(CNN) -- Although the United States' preterm birth rate decreased for the sixth consecutive year, a new report gives the nation a "C" grade.
The 2012 preterm birth rate dropped to 11.5%, a 15-year low, according to the March of Dimes Premature Birth Report Card. Preterm or premature births are defined as birth before 37 weeks of gestation.
However, the report cautions, America's preterm birth rate remains the highest among industrialized countries.
Preterm birth is the leading cause of newborn death, and babies who survive often face serious and sometimes lifelong health problems such as developmental delays or vision loss, according to the organization.
"A premature birth costs businesses about 12 times as much as uncomplicated healthy birth," March of Dimes President Dr. Jennifer L. Howse said in a statement. "As a result, premature birth is a major driver of health insurance costs, not only for employers."
The preterm birth rate peaked at 12.8% in 2006 after rising for more than two decades, according to the March of Dimes. The 2012 rate represents a 10% drop and the best rate since 1998.
Compared with 2006, nearly all states had lower preterm birth rates in 2012.
Thirty-one states, Washington and Puerto Rico all saw improvements. Six states -- Alaska, California, Oregon, Vermont, New Hampshire and Maine -- received an "A" grade. Three states -- Alabama, Mississippi and Louisiana -- received an "F," along with Puerto Rico.
The state with the lowest preterm birth rate was Vermont, with 8.7%. Mississippi was the highest, with 17.1%.
But the March of Dimes said California's success is noteworthy, as the state sees half a million births each year -- the most of any state -- and "has a racially diverse population in a mix of urban, suburban and rural communities that have a variety of healthcare and economic needs."
The organization compared birth rates with its goal of 9.6% by 2020.
Contributing factors were also tracked. For instance, 37 states, along with Washington and Puerto Rico, reduced the percentage of uninsured women of childbearing age. Thirty-five states and Washington reduced the percentage of women of childbearing age who smoked, and 28 states, Washington and Puerto Rico lowered the rate of late preterm births: infants born between 34 and 36 weeks gestation.