Trends in spontaneous and indicated preterm delivery among singleton gestations in the United States, 2005-2012

For the first time in decades, the rate of U.S. preterm delivery has declined consistently since 2005. Recent nationwide policies enforcing elective delivery at or beyond 39 weeks of gestation suggest this decrease may be the result of changes in practice patterns; however, this is not known. Thus, we sought to evaluate whether the decline in preterm delivery was the result of a decrease in indicated or spontaneous preterm delivery and to assess this decrease by race and ethnicity.

This was a population-based retrospective analysis using U.S. vital statistics data restricted to singleton live births from 2005 to 2012. The main outcome measures were overall, indicated, and spontaneous preterm delivery rates. Preterm deliveries were defined as births from 24 to 36 weeks of gestation. We used an algorithm to designate births as indicated or spontaneous. Gestational age was further grouped into early preterm (24-31 weeks of gestation), moderate preterm (32-34 weeks of gestation), late preterm (34-36 weeks of gestation), early term (37-38 weeks of gestation), full term (39-40 weeks of gestation), late term (41 weeks of gestation), and postterm (42-44 weeks of gestation). Analyses were based on the best obstetric estimate of gestational age.

Of 19,984,436 included births, the spontaneous preterm delivery rate declined by 15.4% between 2005 (5.3%) and 2012 (4.5%), whereas indicated preterm delivery rates declined by 17.2% (3.9 to 3.2%). The largest decline was in the postterm pregnancies (-38.5%) followed by early term (-19.1%), early preterm (-17.1%), moderate preterm (-12.4%), and late preterm (-15.8%) with concurrent increases in full term (+14.3%) and late term (+18.7%) gestations. The patterns were similar across race groups.

The noted decline in preterm delivery rates is accompanied by a concurrent decline in both spontaneous and indicated preterm deliveries of almost equal magnitude.