Background: Severe shoulder dystocia is disproportionately associated with large-for-gestational-age infants.
Case: A nulliparous patient at 38 weeks' gestation had an uncomplicated antenatal course. Clinical pelvimetry revealed an acute-angle pubic arch but otherwise normal diameters, conjugate and sacral concavity. Pre-pregnancy BMI was 20.8 and she had had a 14-pound (6.4 kg) weight gain. She presented in labor and, with oxytocin augmentation, progressed to full dilation over 6 hours, followed by an 18-minute second stage. Severe shoulder dystocia was encountered, necessitating multiple maneuvers, and was resolved after 2 minutes with delivery of the posterior arm. The healthy infant weighed 2,289 g (< 5th percentile) and exhibited only transient shoulder weakness, which resolved completely within 1 hour of life. With informed consent, CT pelvimetry was performed within 24 hours postpartum for investigative purposes, revealing small pelvic inlet and at-threshold interischial diameter.
Conclusion: Geometric analysis reveals that borderline adequate pelvimetry likely played a significant role in severe shoulder dystocia etiology, even with a small-for-gestational-age infant. We alert obstetric providers to the possibility of severe shoulder dystocia in patients with borderline adequate pelves on clinical examination, even when estimated fetal weight makes cephalopelvic disproportion unlikely.