WEDNESDAY, Nov. 17 (HealthDay News) -- Forty states and the District of Columbia made progress in reducing their rate of premature births, but the improvements were too small to raise the overall U.S. score from a near-failing 'D' grade, a new report shows.

Each year, the March of Dimes ranks each state according to its rate of premature births, defined as babies born before 37 weeks' gestation.

Premature birth is the leading cause of newborn death. Babies born too early are also at risk of lifelong difficulties such as cerebral palsy and developmental disabilities.

The U.S. preterm birth rate ticked down to 12.3 percent in 2008 (the most recent data available) from 12.7 percent in 2007, according to the report. It was the second decrease in as many years -- in 2006, the premature birth rate was 12.8 percent.

The rate is still substantially higher than the goal of 7.6 percent set by the federal government's Healthy People 2010 campaign, but the declines are an improvement nonetheless, said Jennifer Howse, president of the March of Dimes.

"We feel we have reached a tipping point in our country around this problem," Howse said. "We are encouraged because many of the programs we have worked on together with our partners are in place and starting to work."

U.S. Surgeon General Dr. Regina Benjamin noted that more can be done, however.

"Over half a million babies are born every year preterm, and it is the leading cause of death among newborns," Benjamin said. "As Surgeon General, prevention is my priority, and we need to prevent that."

Benjamin noted that preterm babies typically suffer from any number of health complications, including cerebral palsy, lung problems, digestion problems and vision problems.

"These things can be prevented," she said.

"One, we can provide insurance for pregnant women. Because we know that having those prenatal check-ups and exams can really make a difference," Benjamin explained.

"The next thing that is very easily preventable is for women not to smoke when they're pregnant," Benjamin added. "Not to smoke at all, but particularly when you're pregnant."

Third, women choosing to have an elective cesarean delivery should hold off until after week 39 of their pregnancy before doing so, "because we know that waiting those extra one or two weeks can allow those baby's brains and lungs to develop more fully," she advised.

"That's a very simple thing to do," Benjamin said. "We think pregnancy is nine months, but it's really 40 weeks, which is a little longer than nine months. So, we all need to start thinking about that, because it can really make a difference in the outcome."

In the March of Dimes report, states were graded on how closely they came to meeting the preterm birth objective.

No state earned an 'A' or a 'B.' Vermont, which has the nation's lowest preterm birth rate at 9.5 percent, fell from a 'B' rating last year to a 'C' rating this year because it's preterm birth rate actually ticked up.

New Hampshire and Idaho were the only other states with a preterm birth rate under 10 percent.

All the rest of the states earned 'C's, 'D's and 'F's. Mississippi topped the list with the highest preterm birth rate (18 percent), followed by Alabama (15.6 percent), District of Columbia (15.5 percent), Louisiana (15.4 percent) and South Carolina (14.3 percent). Puerto Rico's preterm birth rate was 19.6 percent.

Despite stubbornly high numbers, most states improved. North Dakota, Kansas, Wyoming and Rhode Island moved up from a 'D' to a 'C'.

Kentucky, which has one of the highest premature birth rates in the nation, dropped from 15.2 percent in 2007 to 14 percent in 2008. Howse credits an extensive public education campaign, "Healthy Babies Are Worth the Wait," done in conjunction with the Kentucky Department for Public Health and the Johnson & Johnson Pediatric Institute for reversing the trend.

The campaign, which is slated to be rolled out in other states, educated women about the risk factors for preterm birth, including smoking, poor nutrition, obesity and medical conditions such as diabetes and hypertension, both of which can be treated during pregnancy to lessen the risk of a premature delivery, Howse said.

And it's not just patients that need educating. Physicians also need to be reminded about the risks of elective early inductions or cesarean sections.

"Women really need support to understand that a pregnancy is 40 weeks, and scheduling an elective induction or C-section prior to that can be quite detrimental to the baby," Howse said.

Studies show even babies born right before 39 weeks -- almost to term but not quite -- are at higher risk of needing to go into the neonatal intensive care unit, are more likely to have feeding problems and respiratory problems, and are more likely to die of sudden infant death syndrome (SIDS) than full-term babies are, Howse said.

"During the last six weeks of the pregnancy, the size of the baby's brain almost doubles," Howse said. "Those weeks are critical for brain development."

The reasons for preterm birth aren't fully understood, but one contributing factor, among many, is the increase in twins and multiples as a result of assisted reproductive technology, Howse noted.

Women who are uninsured and get inadequate prenatal care are also at higher risk of having a premature baby.

About 543,000 babies, or one in eight, are born prematurely each year in the United States, according to the March of Dimes. Costs associated with preterm birth are more than $26 billion annually, according to the Institute of Medicine.

More information

The March of Dimes has more about preventing premature births.