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 for this term to be applicable. Shoulder dystocia happens when the fetal anterior shoulder impacts on the maternal pubic symphysis or, less commonly, the fetal posterior shoulder impacts on the maternal sacral promontory. Additional obstetric maneuvers are often required to help deliver the fetal shoulders.
Incidence of Shoulder Dystocia
The general incidence of shoulder dystocia is between 0.6 and 1.4 percent. The wide variation in incidence is a result of varied clinical scenarios used to describe shoulder dystocia. The definition of shoulder dystocia, the characteristics of the population being examined, and the consistency and accuracy of reporting such cases all affect the reported incidence. 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 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 using 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 result is the inability of the remainder of the body 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. 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. The hairy scalp slides out with reluctance. When the forehead has appeared it is necessary to press back the perineum to deliver the face. Fat cheeks eventually emerge. A double chin has to be hooked over the posterior vulval ...