Aside from multiple births and the risks of medically assisted reproduction, the March of Dimes points to other factors in premature births such as mothers smoking during pregnancy and access to prenatal care.

Dr. Jennifer L. Howse, president of the March of Dimes, points to four key ways to reduce the preterm birthrate that the NCHS report says could cut the infant mortality rate by one-third.

• Improve access to prenatal healthcare to address such chronic conditions as obesity and high blood pressure
• Continue efforts to curb smoking by pregnant mothers. "Twenty percent of preterm births are associated with smoking," Dr. Howse says.
• Prevent unnecessary early inductions and C-sections.
• Encourage use of progesterone hormone supplements by eligible women. The March of Dimes says progesterone can help delay preterm labor in some women.

Despite the low performance grades around the country, Dr. Howse says progress is being made. Nine states—Arizona, Indiana, Missouri, Idaho, Massachusetts, Utah, Wisconsin, Ohio and Oklahoma—have improved their grades since 2008.

"We did see a very small decline in the preterm birth rate, from 12.8 to 12.7 percent of live births in 2007. Also, fewer women of childbearing age are smoking, and the nation's late preterm birth rate also declined, although very, very slightly," she says. "I am optimistic that as we see reductions in the risk factors, we will see a reduction in the rate of preterm birth."