The preterm birth rate in the United States has declined for the fifth year in a row, hitting its lowest level in a decade, according to the March of Dimes, the organization that tracks prematurity across the country.
The 2011 national preterm birth rate was 11.7 percent -- earning the U.S. a "C" on the nonprofit's latest annual report card, released Tuesday.
"We're making progress, but we still have more work to do," Janis Biermann, the March of Dimes' vice president for education and health promotion, told The Huffington Post. "We have some states that are doing well, and we have some states that still have some real challenges."
Vermont, Oregon, New Hampshire and Maine were the only states to earn an "A" on the new report card -- a grade reserved for areas with a preterm birth rate of 9.6 percent or lower -- while 13 states improved by at least one letter grade. Three states -- Louisiana, Mississippi and Alabama -- each earned an "F" on the report card, given preterm birth rates of 14.6 percent or higher in those states.
The March of Dimes has established a goal of lowering the national preterm birth rate to 9.6 percent by the year 2020.
"Prematurity is a very complex issue," Biermann said. "There's not just one cause -- there are multiple causes. Every state needs to be looking at their data and trying to figure out what is going on."
Any baby born before the 37th week of pregnancy is considered premature, and although not every preterm infant has health issues, babies born early are at greater risk for various short and long-term health and developmental issues. Preterm birth is the most frequent cause of infant death in the U.S. and costs the health care system more than $26 billion annually, according to the Centers for Disease Control and Prevention. Generally, the earlier a baby is born, the greater his or her risk of health complications.
Many factors can contribute to the risk of preterm birth, according to Biermann, including -- but not limited to -- access to prenatal care and smoking during pregnancy. Those, she said, are specific factors state health officials can target in order to continue to improve rates in coming years.
According to the March of Dimes' latest report card, the most significant improvements in premature birth rates were among late preterm babies, or those born between 34 and 36 weeks. It dropped from 8.7 to 8.3 percent.
But the U.S. still lags behind other countries in preterm birth rates, ranking 131 out of 184 countries, according to a May report compiled by some 50 health organizations, including the March of Dimes and the World Health Organization. Globally, nearly 15 million infants are born premature each year.
"Any improvement is significant as prematurity is a major cause of newborn death and lifetime handicap," said Dr. Roger Harms, an OB/GYN with the Mayo Clinic and editor-in-chief of the Mayo Clinic Guide to a Healthy Pregnancy.
"It has been very hard to make advances in this area," he said. "It's great to see some progress."
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