A new study has found that the rate of preterm birth in the US has declined for the first time in more than two decades. The study authors, from Columbia University in New York, NY, set out to determine the underlying factors in this decrease. The study was published in the December edition of the journal Obstetrics & Gynecology.
The authors note that the preterm delivery rate has consistently declined every year since 2005 in the US. However, despite this decline, the proportion of women who deliver preterm remains high, compared with other developed nations. They explain that there are two underlying causes of preterm delivery: (1) indicated preterm delivery, which results after labor is induced or a cesarean section performed for maternal or fetal indications; or (2) spontaneous preterm delivery, which can be divided into preterm labor with intact membranes and preterm premature rupture of membranes.
The authors explain that recent nationwide policies imposing elective delivery at or beyond 39 weeks of pregnancy suggest that the decrease in preterm delivery may be due to changes in practice patterns encouraging pregnancy prolongation whenever possible; thus, leading to a decline in indicated preterm deliveries. However, another reason may be the introduction of hormonal (progesterone) therapies to decrease the rate of recurrent preterm delivery, which may have led to a decline in spontaneous preterm deliveries. Determining the reasons for the overall decline may help reinforce effective clinical practices and hopefully continue this downward trend. In addition, it is well-known that race and ethnic disparities are related to preterm delivery. However, the decrease in preterm delivery rates within race and ethnicity subgroups also remains poorly analyzed.
In view of the foregoing, the investigators attempted to determine the decline in preterm delivery in women with singleton gestations (one infant) by indication for preterm delivery and by race and ethnicity. The conducted a retrospective review (backward-looking) using US vital statistics data restricted to singleton live births from 2005 to 2012. The main outcome measures were the overall, indicated, and spontaneous preterm delivery rates. Preterm deliveries were defined as those that occurred from 24 to 36 weeks of pregnancy. They determined whether the preterm births were indicated or spontaneous. Gestational age was divided 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 post-term (42–44 weeks of gestation). The assessments were based on the best obstetric estimate of gestational age.
The researchers found that of 19,984,436 included births, the spontaneous preterm delivery rate declined by 15.4% from 2005 (5.3%) to 2012 (4.5%); indicated preterm delivery rates declined by 17.2% (3.9 to 3.2%) during the same period. The largest decline was in the post-term pregnancies (−38.5%), followed by early term (−19.1%), early preterm (−17.1%), moderate preterm (−12.4%), and late preterm (−15.8%) with simultaneous increases in full term (+14.3%) and late term (+18.7%) gestations. The patterns were similar for all races and ethnicities. The authors concluded that the decrease in preterm delivery rates is accompanied by a simultaneous decrease in both spontaneous and indicated preterm deliveries of almost equal amounts.