• Caesarean delivery;
  • labour induction;
  • preterm birth

Please cite this paper as: Zhang X, Kramer M. The rise in singleton preterm births in the USA: the impact of labour induction. BJOG 2012;119:1309–1315.

Objective  To assess the extent to which increased rates of labour induction and caesarean section have contributed to the recent rise in preterm birth.

Design  National birth cohort study.

Setting  USA.

Population and sample  Singleton live births, with primary analysis based on non-Hispanic white women.

Methods  Ecological study based on the 50 states and the District of Columbia during two time periods 10 years apart: 1992–94 and 2002–04.

Main outcome measure  Preterm birth (live birth <37 completed weeks of gestation), based on an algorithm combining menstrual and clinical estimates of gestational age.

Results  The state-level ecological analysis among non-Hispanic white women showed that the change in preterm birth rate from 1992–94 to 2002–04 was significantly associated with the change in rate of labour induction (r = 0.50, 95% CI 0.26–0.68), but not with the change in rate of caesarean delivery (r = −0.06, 95% CI −0.33 to 0.22). Weaker but otherwise similar associations with labour induction were observed in Hispanic women and in non-Hispanic black women.

Conclusions  Increasing use of labour induction is probably an important cause of the observed increased rate in preterm birth.