Association of fetal cranial shape with shoulder dystocia.

OBJECTIVE

To evaluate whether fetal cranial shape is related to shoulder dystocia.

METHODS

We compared shoulder dystocia cases (n = 18) with controls (normal vaginal deliveries, n = 18) in a retrospective matched-pairs observational study. Subjects were matched for known maternal and fetal risk factors and then evaluated for fetal biometric differences, which were measured by ultrasound near delivery. We tested multivariable risk models to predict shoulder dystocia by logistic regression.

RESULTS

Cases had a smaller estimated occipitofrontal diameter (OFD) (P = 0.02) and a larger biparietal diameter/estimated OFD ratio (P = 0.003). A multivariable model including estimated fetal weight, estimated OFD, maternal weight and diabetes mellitus had sensitivity and specificity of 86% and 95%, respectively, and positive and negative likelihood ratios of 18.9 and 0.15, respectively. Estimated OFD significantly increased the predictive value of the model.

CONCLUSION

A small estimated OFD is a risk factor for shoulder dystocia in the presence of other significant risk factors. A multivariable model including estimated OFD can predict shoulder dystocia in a clinically useful range.