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The Association between Obstetrical Interventions and Late Preterm Birth

Kate L. Bassil, PhD, Abdool S. Yasseen III, MSc, Mark Walker, MD, Michael D. Sgro, MD, Prakesh S. Shah, MD, Graeme N. Smith, MD, PhD, Douglas M. Campbell, MD, Muhammad Mamdani, PharmD, Ann E. Sprague, PhD, Shoo K. Lee, MDa

American Journal of Obstetrics and Gynecology, In Press, February 2014

Editor’s comment: Prof. Jim Thornton: Late pre-term birth not associated with obstetric interventions
Rates of late pre-term birth (34-36 weeks) are increasing. This may be due to increased risks, such as multiple pregnancy, obesity and advanced maternal age, or to increased obstetric intervention. In this large population-based study from Canada, after adjustment for clinical risk factors, late preterm births were significantly less likely to have undergone labour induction or pre-labour Caesarean section. This suggests that obstetricians are preferentially avoiding induction and pre-labour Caesarean section between 34 and 37 weeks gestation.



There is concern that obstetric interventions (prelabor cesarean section and induced delivery) are drivers of late preterm (LP) birth. Our objective was to evaluate the independent association between obstetric interventions and LP birth and explore associated independent maternal and fetal risk factors for LP birth.

Study Design

In this population-based cross-sectional study, the BORN Information System was used to identify all infants born between 34 and 40 completed weeks of gestation between 2005 and 2012 in Ontario, Canada. The association between obstetric interventions (preterm cesarean section and induced delivery) and LP birth (34 to 36 completed weeks' gestation vs 37 to 40 completed weeks' gestation) was assessed using generalized estimating equation regression.


Of 917,013 births between 34 and 40 weeks, 49,157 were LP (5.4%). In the adjusted analysis, “any obstetric intervention” (risk ratio [RR], 0.65; 95% confidence interval [CI], 0.57–0.74), induction (RR, 0.71; 95% CI, 0.61–0.82) and prelabor cesarean section (RR, 0.66; 95% CI, 0.59–0.74) were all associated with a lower likelihood of LP vs term birth. Several independent potentially modifiable risk factors for LP birth were identified including previous cesarean section (RR, 1.28; 95% CI, 1.16–1.40), smoking during pregnancy (RR, 1.28; 95% CI, 1.21–1.36) and high material (RR, 1.1; 95% CI, 1.03–1.18) and social (RR, 1.09; 95% CI, 1.02–1.16) deprivation indices.


After accounting for differences in maternal and fetal risk, LP births had a 35% lower likelihood of obstetric interventions than term births. Obstetric care providers may be preferentially avoiding induction and prelabor cesarean section between 34 and 37 weeks' gestation.


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Commentaries by Editor Prof. Jim Thornton