May 11, 2010 -- After a long period of fairly steady increase, the preterm birth rate in the U.S. has declined for the second straight year, investigators found.
In 2008, the rate was 12.3 percent of all births being premature, down from 12.7 percent in 2007 and 12.8 percent in 2006, Joyce A. Martin, of the U.S. Centers for Disease Control, and colleagues reported in a statistical brief.
"It's a very encouraging decline, and we're hopeful that it portends the beginning of a long-term downward trend, but it's too early to tell if that's the case," Martin told MedPage Today.
The new numbers mark the first two-year decline in nearly three decades, Martin said.
Between 1980 and 2006, rates of premature births rose by more than a third.
The researchers speculated that the increase may have been driven by greater use of interventions such as inducing labor and cesarean delivery earlier in pregnancy.
More recently, they wrote, obstetricians may be putting a halt on early use of those procedures.
"I think it is very likely that this decrease relates to the widely publicized criticism of delivering babies 'early' for a laundry list of poorly-defended indications," Dr. Ian Holzman, chief of newborn medicine at Mount Sinai Medical School in New York City, said in an e-mail.
"We are finally not scheduling inductions for convenience at less than 39 weeks," he said. "Similarly, cesarean sections don't occur early unless there is a strong medical indication."
Holzman, who was not involved in the study, added that there's likely also better control of multiple gestations via reproductive technology, as multiples have higher preterm rates than singletons.
But Martin noted that declines in preterm birth occurred across all categories, even among non-induced births.
"Fewer interventions might be influencing the decline, but the fact that it occurred among non-induced births is very interesting," she said. "It's a very complex puzzle."
Declines in preterm births were also seen across the span of pregnancy. In 2008, the percentage of infants born late preterm -- 34 to 36 weeks gestation -- fell by 0.3 percent, to 8.8 percent from 9.1 percent in 2006. The late preterm rate had risen 25 percent between 1990 and 2006.
Similarly, those delivered early preterm -- less than 34 weeks gestation -- fell to 3.6 percent in 2008 compared with 3.7 percent in 2006. That was up from 3.3 percent in 1990.
Although preterm birth rates dropped among both blacks and whites, blacks are still more likely to give birth preterm.
Among whites, the preterm birth rate fell from 11.7 percent in 2006 to 11.1 percent in 2008. For blacks, rates fell to 17.5 percent in 2008, from 18.5 percent in 2006.
Among Hispanics, rates fluctuated -- but generally rose -- between 1990 and 2007. Yet they fell from 12.3 percent at the end of that period to 12.1 percent in 2008.
Martin and colleagues noted that "large differences [are] still evident among race and Hispanic origin."
But there's less difference by age. Preterm birth rates were down among women of all age groups under 40 -- declining by 3 percent to 5 percent among women under 35, and by 2 percent for those aged 35 to 39.
Changes for women over 40 weren't statistically significant.
Rates also declined across the country, with only Hawaii reporting an increase in preterm births.
Martin said the reduction is not explained by changes in the number of multiple births, even though multiples are traditionally at greater risk of preterm delivery than singletons.
As Holzman noted, research has shown that increases in preterm induction of labor and cesarean delivery contributed to the initial rise in the overall preterm birth rate. Decreases in use of these procedures before 37 weeks may have reduced this rate, the researchers wrote.
Still, they cautioned that although the current preterm birth rate dropped slightly, it is still higher than in any year from 1981 to 2002, and further research is needed "to explain the factors behind the current downturn and develop approaches to help ensure its continued decline."