Shoulder dystocia (SD) is an acute and unpredictable obstetric event occurring during a vaginal birth after the baby’s head has been delivered. It is defined by the inability of the baby’s shoulders to pass through the mother’s pelvis without specific, urgent maneuvers. This complication arises when the baby’s anterior shoulder becomes lodged behind the mother’s pubic bone, preventing the rest of the body from emerging. Because the baby’s chest is compressed, immediate action is required to safely complete the delivery.
Defining Shoulder Dystocia and Its Frequency
Shoulder dystocia is formally diagnosed when the delivery of the fetal shoulders requires additional obstetric manipulations after gentle downward traction has failed. The physical obstruction occurs when the anterior shoulder cannot pass beneath the pubic symphysis. A telltale sign is the “turtle sign,” where the baby’s head retracts back toward the perineum shortly after delivery.
The frequency of shoulder dystocia in vaginal deliveries is relatively low, typically ranging between 0.4% and 1.5%. Although the vast majority of deliveries are unaffected, the unpredictable nature of the event requires medical teams to always be prepared. Some reports place the incidence slightly higher, up to 3.0%, depending on how the condition is defined and reported. The actual occurrence rate is influenced by the characteristics of the birthing population, such as the prevalence of certain maternal health conditions.
Factors Increasing the Likelihood
Although shoulder dystocia can occur without identifiable risk factors, several elements increase its probability. Fetal macrosomia, or a baby with an unusually large size, is a major predisposing factor. The risk increases when the estimated fetal weight is over 4,000 grams (about 8 pounds, 13 ounces), and rises further for babies weighing over 4,500 grams.
Maternal diabetes, whether pre-existing or gestational, independently raises the risk because it often leads to disproportionate fetal growth. Babies of diabetic mothers tend to have excess fat distribution on their shoulders and trunk, making the shoulder diameter relatively wider compared to the head. A history of a previous shoulder dystocia delivery increases the recurrence risk in a subsequent vaginal birth by up to ten times the general population rate.
Labor factors can also contribute to the likelihood of this event. These include a prolonged second stage of labor or the need for an assisted vaginal delivery using instruments like forceps or a vacuum extractor. Recognizing these pre-delivery factors allows for heightened vigilance, though they cannot predict every instance of shoulder dystocia.
Emergency Maneuvers Used During Delivery
Once shoulder dystocia is identified, medical professionals follow a sequential protocol of maneuvers to dislodge the impacted shoulder quickly. The clock starts ticking when the head delivers, as prolonged compression can lead to oxygen deprivation for the baby. The first and least invasive step is the McRoberts maneuver, which involves sharply hyperflexing the mother’s thighs up toward her abdomen.
This repositioning technique flattens the sacrum and rotates the pubic symphysis, effectively increasing the space in the pelvic outlet. Simultaneously, suprapubic pressure is applied by an assistant just above the pubic bone to help rotate the anterior shoulder beneath the symphysis. Pressure applied to the uterine fundus is avoided, as it can worsen the impaction.
If these external steps fail, internal rotational maneuvers are attempted to manually turn the baby’s shoulder girdle. The Woods Screw maneuver involves applying pressure on the anterior aspect of the baby’s posterior shoulder, rotating the baby in a corkscrew fashion. Conversely, the Rubin maneuver involves pushing on the posterior aspect of the anterior shoulder to rotate the shoulder girdle out of the narrowest diameter of the pelvis.
Potential Complications for Mother and Baby
The physical force required to resolve shoulder dystocia can lead to complications for both the baby and the mother. For the baby, the most common injury is a Brachial Plexus Injury (BPI), which is damage to the network of nerves controlling arm movement and sensation. While most BPIs are temporary, a small percentage can result in permanent weakness or paralysis.
The maneuvers may also result in fractures, most commonly to the clavicle or the humerus. These fractures heal well without lasting issues, but they are a direct consequence of the physical strain necessary to free the baby. If the delay in delivery extends beyond five minutes, the risk of hypoxic ischemic encephalopathy, or brain injury due to lack of oxygen, significantly increases.
For the mother, the primary risks relate to soft tissue trauma and excessive blood loss. The need for rapid delivery and manipulation can lead to severe vaginal and perineal lacerations, sometimes extending to the anal sphincter. This disruption can also increase the risk of postpartum hemorrhage (PPH), which is a significant loss of blood after delivery.