THURSDAY, Nov. 6, 2014 (HealthDay News) -- Preterm births in the United States fell to 11.4 percent in 2013, the lowest rate in 17 years, the March of Dimes reported Thursday.
And an unrelated U.S. study finds more good news: Since 2005, the rate of preterm deliveries has declined consistently each year for the first time in more than two decades.
However, experts hope to see the number of premature births fall even lower.
"Having a preterm baby increases risks of complications, a long stay in the neonatal intensive care unit, respiratory problems, jaundice, difficulties with breast-feeding and, later, developmental differences perhaps and developmental delays," said Dr. Cynthia Gyamfi-Bannerman, associate professor of obstetrics and gynecology at Columbia University Medical Center in New York City.
She looked at U.S. vital statistics data on nearly 20 million single, live births from 2005 to 2012. The spontaneous preterm delivery rate fell more than 15 percent during those years, reaching about 4.5 percent two years ago, she said.
Spontaneous preterm delivery refers to a baby born unexpectedly early.
Meanwhile, the rate of indicated preterm delivery -- meaning labor is induced, perhaps because the mother has dangerously high blood pressure -- fell from 3.9 to 3.2 percent in that time period. That represents a decline of more than 17 percent.
"We were actually surprised that we found both of them had gone down pretty significantly," said Gyamfi-Bannerman.
For the study, published online Nov. 6 in Obstetrics & Gynecology, she defined preterm deliveries as occurring from 24 to 36 weeks and early term as 37 and 38 weeks. Full-term was 39 or 40 weeks.
Despite noteworthy progress, the March of Dimes' annual report card on premature births awarded the nation a "C" grade.
That's because even though it met the federal Healthy People 2020 goal seven years early, it fell short of the more ambitious 9.6 percent goal set by the March of Dimes, which works to improve the health of mothers and babies.
Overall, the U.S. rate of preterm births is one of the highest among wealthy nations, the agency noted.
In 2013, more than 450,000 babies were born early, compared to more than 542,000 in 2006, when preemie rates peaked, the report said.
Three states -- Louisiana, Mississippi and Alabama -- plus Puerto Rico received a failing grade because of preterm birth rates of 14.6 percent or more.
However, five states with a rate of 9.6 percent or lower got an "A." They are California, Oregon, Maine, New Hampshire and Vermont.
"We are improving," said Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in New York City, who wasn't involved in either report. But "the fact that some states got an F? That's very bad," she said. "It's something we need to work on."
Wu attributed some of the drop in preterm births to changing medical practices. Doctors and hospitals now emphasize the importance of waiting until at least 39 weeks before inducing delivery, she said.
"If you go into labor at 37 weeks, naturally, that is considered a full-term delivery," she said. But doctors are aiming to reduce elective deliveries before 39 weeks, she added.
The practice of giving progesterone to women with a history of preterm birth has also helped reduce preterm deliveries, Gyamfi-Bannerman said.
Good health care can help mothers-to-be boost their chances of a full-term delivery, these experts suggested.
Pregnant women should not smoke, drink alcohol or engage in other risky behaviors such as drug use, Wu said.
Also, women who've had a preterm baby are at higher risk of delivering early again, Gyamfi-Bannerman said. They should see a specialist with expertise in treating women with a history of preterm births, she advised.
Spacing pregnancies can also reduce risk, research suggests. An interval of at least six months is recommended, Gyamfi-Bannerman said.
Wu added: "The best pregnancies are planned pregnancies."
To see your state's premature birth rate, visit the March of Dimes.