May 11, 2010 -- Though premature births rose dramatically from the early 1980s through 2006, new research suggests a change in that long upward trend, with the preterm birth rate declining in 2007 and again in 2008.
It is the first two-year downturn in nearly 30 years, the CDC says in a new report.
The preterm birth rate declined in 2008 to 12.3%, down 3% from 2007, when it was 12.7%. The 2008 preterm birth rate was 4% below the 12.8% peak recorded in 2006.
CDC says the preterm birth rate rose more than 20% between 1990 and 2006.
Preterm rates were lower for births to women of all age groups under 40 in 2008 compared with 2006, the CDC’s National Center for Health Statistics reports.
The long rise in the preterm birth rate “has been a cause of great concern” because babies born early are at increased risk of early death or disability, the CDC says.
Normal pregnancy lasts about 40 weeks. Preterm is defined as less than 37 weeks of gestation, late preterm 34 to 36 weeks, and early preterm less than 34 weeks.
The report says that many reasons, including maternal demographics and increases in multiple births, have been cited as causes for the rising preterm birth rate. The increased use of obstetric interventions, such as labor induction and cesarean delivery, also have been mentioned as probable causes of the long upturn in early births.
The new report indicates it is not clear what caused the two-year decline. It is not explained by changes in the proportions of multiple births, and preterm rates among single births declined in 2007-2008.
“This report finds that the decline in the overall preterm rate from 2006 to 2008 was related to declines in all types of delivery -- that is, in preterm cesarean and induced and non-induced vaginal births,” it says. “Further research is necessary to explain the factors behind the current downturn and to develop approaches to help ensure its continued decline.”
Among many major findings, the report says that:
- Preterm birth rates declined from 2006 to 2008 for moms of all age groups under age 40, and for most U.S. states.
- The percentage of preterm births was down for all types of deliveries, including cesarean deliveries and induced labor.
- The percentage of babies born late preterm, or between 34 and 36 weeks gestation, declined from 9.1% in 2006 to 8.8%. This rate had risen 25% between 1990 and 2006.
- The percentage of newborns delivered early preterm was down to 3.6% in 2008 compared with 3.7% in 2006. Infants delivered in early preterm are much more likely to suffer long-term disability or early death, compared to babies born after 34 weeks.
- Preterm birth rates declined 3% to 5% among women under 35 and by 2% for women 35-39.
- The 2008 preterm rate for non-Hispanic whites was 11.1%, down from 11.5% in 2007 and from 11.7% in 2006. Preterm births among this group had climbed 38% in the 1990-2006 period.
- For non-Hispanic blacks, the preterm birth rate declined to 17.5% in 2008 from 18.5% in 2006.
- For Hispanic infants, the birth rate declined from 12.3% to 12.1% from 2007 to 2008.
The report says preterm births have declined significantly in 35 states since 2006, with only Hawaii reporting an increase.
In 2006-2008, the percentage of all cesarean births that were preterm declined 4% to 17.1%. The percentage of induced vaginal births born preterm also decreased in the period, to 7.2% from 7.7%. The percentage of noninduced vaginal births dropped from 11.6% to 11.1%.