Shoulder dystocia is an obstetrical emergency that can occur during vaginal delivery, and it refers to when the fetal shoulders can’t progress past the maternal pubic bone after the fetal head has been delivered. This usually requires additional obstetrical maneuvers to deliver the baby safely.
Let’s start by looking at the anatomy and physiology of the birth canal during delivery, starting with the pelvis, made up of the sacrum and the paired hip bones, each of which is made up of the ilium, ischium and pubic bones. The anatomy of the female bony pelvis differs from the male bony pelvis, in that it’s wider and flatter in shape and has a wide pubic arch, which helps with vaginal delivery. Usually, the female pelvic inlet is about 12cm in an antero-posterior diameter, and 13cm in a transverse diameter.
Now, remember that the bisacromial diameter, meaning the distance between the outermost parts of the fetal shoulders, is usually between 12 and 15 centimeters. Because of that, the fetal shoulders typically enter the pelvis at an oblique angle, and the posterior shoulder, meaning the one facing the back of the birth canal, is usually a little ahead. Following external rotation of the fetal head, the anterior shoulder can usually glide under the symphysis pubis in order to be delivered.
Now, shoulder dystocia is usually caused by discrepancy between fetal shoulders and maternal bony pelvis. So, risk factors for shoulder dystocia include maternal factors, which can be remembered with the acronym SAP-DOE. This stands for small pelvic inlet; advanced age; prior history of shoulder dystocia or large infants; gestational or pregestational diabetes, obesity, and excessive maternal weight gain. The main risk factor related to the fetus is fetal macrosomia, or a fetus that’s larger than normal. Finally, there are risk factors related to the labor process, which include a prolonged second stage of labor, abnormal labor progress, and labor induction. Now, pathology-wise, during delivery, the anterior fetal shoulder can become impacted behind the symphysis pubis or the posterior shoulder can be obstructed by the maternal sacrum bone.