Pregnancy week at delivery and the risk of shoulder dystocia: a population study of 2 014 956 deliveries

ObjectiveTo study whether pregnancy week at delivery is an independent risk factor for shoulder dystocia. DesignPopulation study. SettingMedical Birth Registry of Norway. PopulationAll vaginal deliveries of singleton offspring in cephalic presentation in Norway during 1967 through 2009 (n=2014956). MethodsThe incidence of shoulder dystocia was calculated according to pregnancy week at delivery. The associations of pregnancy week at delivery with shoulder dystocia were estimated as crude and adjusted odds ratios using logistic regression analyses. We repeated the analyses in pregnancies with and without maternal diabetes. Main outcome measuresShoulder dystocia at delivery. ResultsThe overall incidence of shoulder dystocia was 0.73% (n=14820), and the incidence increased by increasing pregnancy week at delivery. Birthweight was strongly associated with shoulder dystocia. After adjustment for birthweight, induction of labour, use of epidural analgesia at delivery, prolonged labour, forceps-assisted and vacuum-assisted delivery, parity, period of delivery and maternal age in multivariable analyses, the adjusted odds ratios for shoulder dystocia were 1.77 (1.42-2.20) for deliveries at 32-35weeks of gestation, and 0.84 (0.79-0.88) at 42-43weeks of gestation, using weeks 40-41 as the reference. In pregnancies affected by diabetes (n=11188), the incidence of shoulder dystocia was 3.95%, and after adjustment for birthweight the adjusted odds ratio for shoulder dystocia was 2.92 (95% CI 1.54-5.52) for deliveries at weeks 32-35 of gestation, and 0.91 (95% CI 0.50-1.66) at 42-43weeks of gestation. ConclusionThe risk of shoulder dystocia was associated with increased birthweight, diabetes, induction of labour, use of epidural analgesia at delivery, prolonged labour, forceps-assisted and vacuum-assisted delivery, parity and period of delivery but not with post-term delivery.