LITTLE ROCK — A new March of Dimes study shows Arkansas’ premature birth rate dropped about 0.5 percent in the last six years, but the downward trend rated the state no better than a "D" on the 2012 March of Dimes Premature Birthrate Card released Tuesday.

Dr. Whit Hall, a neonatologist at the University of Arkansas for Medical Sciences and the 2013 March of Dimes Citizen of the Year, said the March of Dimes and the state Department of Health recently formed a partnership improve the number of healthy births in Arkansas.

Under the March of Dimes’ grading system, states with premature birth rates of 9.6 percent or less were given an A; between 9.6 percent and 11.3 percent a B; 11.3 percent to 12.9 percent a C; 12.9 percent to 13.6 percent a D, and above 13.6 percent an F.

New Hampshire, Maine, Oregon and Vermont were the only states to receive an A. Arkansas, along with Alabama and Mississippi, received a D.

Janalyn Williams, state director of the March of Dimes, said that a baby is considered premature if he or she is born less than 37 weeks into the mother’s pregnancy.

The state’s premature birth rate in 2006 was 13.7 percent. It dropped to 12.7 percent in 2010 but rose to 13.2 percent in 2011, the latest year for the statistics. About 37,000 babies are born each year in the state, officials said.

"That’s about 1,800 babies that have not been born premature by having that decrease," Williams said. "Even though it’s a slight decrease, it’s still a measurable amount of babies that are born (full-term) each year."

Williams said research is under way to understand why the premature birth rate percentage rose from 12.7 percent in 2010 to 13.2 percent last year.

"We don’t know why this is happening but certainly it’s a problem," she said.

She added that the state’s premature birth rate is high — the rate is 11.7 percent nationally — because many pregnant women have little or no access to health care.

"Many of those women don’t know that they need regular and quality health care during pregnancy," she said.

Another contributing factor is smoking, Williams said, adding that 25 percent of pregnant women in Arkansas smoke during their last trimester of pregnancy.