Maternal body mass index and fetal weight in shoulder dystocia


      Excessive body mass index (BMI) and increased fetal weight are independent risk factors for shoulder dystocia (SD). We explored the effect of combining BMI and fetal weight as they may have a synergistic effect on risk of SD.

      Study design

      All SD cases from 1997-2001 at a single center were identified and matched 1:1 based on parity, race, birthweight, and induction to a control population who delivered vaginally without SD. Statistical analysis included paired t tests, Wilcoxon rank sum, and chi-square. The subset of babies with SD and early neonatal neurological injury (brachial palsy or clavicular/humeral fracture) was analyzed separately. BMI, fetal weight, and an index multiplying BMI and fetal weight were compared between the two groups.


      There were 24,094 deliveries and 207 (0.8%) cases of shoulder dystocia, consisting of 65 nulliparous and 142 multiparous patients. The incidence of neonatal neurological injury was 27.7% in the nulliparous group compared to 15.9% in the multiparous group (P = 0.04). In nullipara, the index of BMI × fetal weight became progressively higher as the severity of cases increased, with the mean value lowest in the control group compared to the SD group (118 vs 133; P = 0.02) and highest in the SD cases with neurologic injury (118 vs 150; P = 0.003). In multipara, the index mean for the control group was similar to that of the SD group (135 vs 127; P = 0.09) and again highest in the SD subset with neurologic injury (135 vs 156; P = 0.05).


      Evaluating SD and its risk factors in a prognostically meaningful and consistent fashion is important. In this small sample of SD cases with neurological impairment, combining the anthropometrical measurement of BMI, which influences birth canal capacity, and fetal weight, a measure of baby size, appears useful.