The rise in singleton preterm births in the USA: the impact of labour induction

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.