Definition of Shoulder Dystocia
Shoulder dystocia occurs when the fetal head is delivered but the shoulders cannot be spontaneously delivered by the usual method of gentle downward traction. The fetus must be in the cephalic presentation. Shoulder dystocia occurs when the anterior fetal shoulder is obstructed by the maternal pubic symphysis or, less commonly, from impaction of the posterior fetal shoulder on the maternal sacral promontory. Additional obstetric maneuvers are required to help deliver the fetal shoulders. Retraction of the delivered head against the maternal perineum, the “turtle sign,” is suggestive of shoulder dystocia but not diagnostic. Shoulder dystocia is an unpredictable and unpreventable obstetrical emergency, thus obstetrical care providers should have the knowledge and ability to perform additional obstetrical maneuvers promptly.
Incidence of Shoulder Dystocia
The general incidence of shoulder dystocia is between 0.2 and 3.0 percent of vaginal births. The variation in reported incidence is due to differences in the definition of shoulder dystocia, characteristics of the study populations, varied clinical scenarios, reliance on the health care provider’s clinical judgment, and the consistency and accuracy of reporting. Approximately 50 percent of shoulder dystocias occur in women without risk factors.
Mechanism of Shoulder Dystocia
In most cases of normal labor and delivery, the fetal shoulders enter the pelvis in an oblique diameter. As labor progresses, the shoulders descend and rotate the bisacromial diameter toward the anteroposterior (AP) diameter of the pelvis. By this mechanism, the anterior shoulder comes under the pubic symphysis a little to the side of the midline and is then delivered.
Impaction of the shoulders occurs when the fetus attempts to enter the pelvis with the bisacromial diameter in the AP diameter of the inlet (Fig. 18-1) instead of one of the oblique diameters. Rarely do both shoulders impact above the pelvic brim. Usually, the posterior shoulder can negotiate its way past the sacral promontory, but the anterior shoulder becomes wedged against the pubic symphysis.
Shoulder dystocia: bisacromial diameter in the anteroposterior diameter of the pelvis.
When the anterior shoulder, or less commonly the posterior shoulder, is impacted against the symphysis pubis/sacral promontory in the AP diameter, the remainder of the body is unable to be delivered by the usual methods. The head remains tight against the perineum (the “turtle sign”), spontaneous restitution does not occur, and the baby does not deliver with the usual maternal effort and gentle downward traction. In 1955, Morris described the classic picture of shoulder dystocia as follows:
The delivery of the head with or without forceps may have been quite easy, but more commonly there has been a little difficulty in completing the extension of the head. ...