Objective: To explore the relationship between maternal obesity and shoulder dystocia while controlling for the potential confounding effects of other variables associated with obesity.
Methods: We performed a case-control study of provincial delivery records audited by the Northern and Central Alberta Perinatal Outreach Program. Risk factors evaluated were selected based on previously published studies. Cases and controls were drawn from 45,877 live singleton cephalic vaginal deliveries weighing more than 2500 g between January 1995 and December 1997. There were 413 cases of shoulder dystocia (0.9% incidence). Controls (n = 845) were randomly chosen from the remainder of the target population to create a 1:2 case/control ratio. Univariate analysis with calculation of odds ratios (ORs) was used to determine which of the chosen risk factors were significantly related to the incidence of shoulder dystocia. Multivariable regression analyses were then used to determine the independently associated variables, and the adjusted ORs were obtained for each relevant risk factor.
Results: Maternal obesity was not significant as an independent risk factor for shoulder dystocia after adjusting for confounding variables (adjusted OR 0.9; 95% confidence interval [CI] 0.5, 1.6). Fetal macrosomia was the single most powerful predictor. The adjusted ORs were 39.5 (95% CI 19.1, 81.4) for birth weight greater than 4500 g and 9.0 (95% CI 6.5, 12.6) for birth weight between 4000 and 4499 g.
Conclusion: The strongest predictors of shoulder dystocia are related to fetal macrosomia. For obese nondiabetic women carrying fetuses whose weights are estimated to be within normal limits, there is no increased risk of shoulder dystocia.