Shoulder Dystocia

Etiology

The underlying cause of shoulder dystocia is the persistent anterior-posterior position of the fetal shoulders as they enter the pelvic inlet or descent of the fetal shoulders simultaneously. According to the American College of Obstetricians and Gynecologists (ACOG), the following are the 3 primary mechanisms of this:

• Increased resistance between the vaginal walls and the fetus (eg, fetal macrosomia)
• A large fetal chest relative to the fetal head (eg, neonates of diabetic women)
• Rapid descent prevents rotation of the bisacromal diameter within the pelvis (eg, a precipitous delivery)[1]

Risk Factors for Shoulder Dystocia

Unfortunately, shoulder dystocia can be challenging to predict and cannot be reliably prevented.[1] The most significant risk factors are maternal diabetes, previous shoulder dystocia, and fetal macrosomia. Maternal diabetes causes elevated glucose levels in the fetus, which stimulates it to produce excess insulin, insulin-like growth factors, and growth hormone, potentially leading to large for gestational age infants or infants with larger shoulders and increased abdominal-to-head circumference ratios.[5] Larger infants may have difficulty traversing the fixed diameters of the maternal pelvis.[6] Despite this, the majority of shoulder dystocias still occur in nondiabetic mothers with average-size infants, while many patients with diabetes or macrosomic infants do not have shoulder dystocias.[1] 

For patients with a history of shoulder dystocia, operative (ie, forceps or vacuum) vaginal delivery, or an infant with a brachial plexus injury in a prior pregnancy, the risk of recurrence is estimated to be ≥10%.[1] For this reason, primary elective cesarean delivery may be considered after careful evaluation of the entire clinical picture.[1]  For patients who elect to proceed with a trial of labor, the delivering team must maintain heightened awareness and prepare for recurrent shoulder dystocia. Prophylactic use of the McRoberts maneuver may be considered in these situations. No advantage, though, has been observed with its use before the appearance of clinical signs suggestive of shoulder dystocia. However, there are minimal risks in using the maneuver prophylactically at the time of delivery.[7]

Additional risk factors for shoulder dystocia include prior operative (ie, forceps or vacuum) vaginal delivery, delivery of a large birthweight infant, increased maternal weight gain, maternal obesity, and intrauterine fetal death. These factors, however, are often present concurrently with other more significant risk factors (eg, fetal macrosomia and maternal diabetes), making it difficult to isolate the individual risk associated with each clinical feature.[8][9] 

Intrapartum conditions that may signal the possibility of impending shoulder dystocia include a prolonged second stage of labor, failure of the fetal head to descend, and need for the operative (ie, forceps or vacuum) vaginal delivery of the fetal head.[2] However, while it is essential to remain mindful of the risk factors for shoulder dystocia to prepare for delivery properly, attempts to predict shoulder dystocia based on risk factors have shown poor reliability and have a poor predictive value.[10] 

Epidemiology

Shoulder dystocia is thought to occur in approximately 0.2% to 3% of all vaginal deliveries in which the fetus is in a vertex presentation.[1][2] The incidence of shoulder dystocia increases as the size of the infant increases. The incidence of shoulder dystocia in deliveries with infants weighing <4,000 g is approximately 1%. However, for deliveries with infants weighing 4,000 to 4,500 g, this increases to about 5% to 9% and rises to approximately 14% to 23% for infants weighing more than 4,500 g.[6][11][12]

Pathophysiology

Familiarity with the boney female pelvis is essential in understanding how shoulder dystocia develops and provides insight into why the correct application of various maneuvers can relieve shoulder dystocia. 

Composition of the Pelvic Girdle

The pelvic girdle is comprised of 4 bones and 3 joints. The bones consist of the sacrum, the coccyx, and the two innominate bones formed by the fusion of the ilium, ischium, and pubic rami.[13] The joints include the sacroiliac joints bilaterally located between the sacrum and each ilium, and the pubic symphysis joint, where the right and left pubic rami meet anteriorly.[13]

Pelvic Spaces: The Lesser Pelvis and Greater Pelvis

The space enclosed by the pelvic girdle has a bowl-like shape and is divided into the greater and the lesser pelvis. The greater pelvis (ie, false pelvis) refers to the superior portion between the iliac wings. The lesser pelvis (ie, true pelvis) lies inferior to the greater pelvis and is located between the sacrum and coccyx posteriorly and the pubic and ischial bones anteriorly and laterally.[14]

A somewhat round, bony ridge known as the pelvic brim is at the boundary between the greater and lesser pelvis; it is comprised of the sacral promontory, the arcuate line on the ilium, and the superior border of the pubic rami. The pelvic brim circumscribes an opening known as the pelvic inlet. The pelvic inlet is thus the "entrance" into the lesser, or true, pelvis. The pelvic outlet is the opening at the lower margin of the lesser pelvis. This margin is made up posteriorly of the coccyx, posterolaterally of the sacrotuberous ligaments, laterally of the ischial tuberosities, and anteriorly of the pubic arch. Thus, during labor and delivery, the fetus must descend through the pelvic inlet, traverse the lesser pelvis, and emerge through the pelvic outlet.[15] Shoulder dystocia occurs when one of the fetal shoulders gets stuck behind the pelvic brim.

Fetal Rotation Through the Lesser Pelvis

The obstetric (ie, true) conjugate is the distance between the sacral promontory and the widest portion of the pubic symphysis, located between the superior and inferior margins of the symphysis. The obstetric conjugate represents the smallest fixed distance through the pelvic inlet.[16] The obstetric conjugate in the average gynaecoid pelvis is approximately 11.5 to 12 cm, while the average transverse diameter is typically slightly larger at 13 cm.[6] The largest diameters of the fetal head are in the A-P plane, while the largest diameter of the fetal shoulders is in the transverse plane. For the largest diameters of the fetal head and shoulders to pass through the larger transverse diameter of the maternal pelvis, the fetus must rotate through the true pelvis.

The fetal head typically passes through the pelvic inlet in a transverse position (ie, facing one of the maternal sides). The fetus then internally rotates, more directly aligning its head and shoulders in an A-P orientation (ie, facing either the maternal abdomen or back), which allows its bisacromial diameter to align with the transverse diameter of the maternal pelvis. The fetal head then extends with delivery and restitutes, realigning the fetal head, shoulders, and rest of the body, which represents an external rotation. In some cases, especially with larger infants, the fetal shoulders get stuck behind the bones of the pelvic inlet while attempting to rotate, resulting in a shoulder dystocia.

Effect of Shoulder Dystocia on Fetal Oxygenation

After delivery of the fetal head, the fetal trunk, including the chest, abdomen, and umbilical cord, becomes compressed within the vaginal canal resulting in decreased fetal oxygenation due to reduced blood flow through the umbilical cord. Additionally, the compression prevents the fetus from expanding its lungs effectively until the body is fully delivered.[6] Consequently, rapid relief of shoulder dystocia to avoid fetal asphyxiation is critical in reducing infant morbidity and mortality.

History and Physical

On admission, a comprehensive maternal history, focused physical examination, and review of all imaging should be obtained in an attempt to identify potential risk factors, such as:

• Maternal diabetes, obesity, and weight gain during pregnancy
• History of shoulder dystocia with a prior delivery
• Estimated fetal weight of 5,000 g in women without diabetes or 4,500 g in women with diabetes [1][2]

During labor and delivery, it is always prudent for the delivering clinician to monitor the position of the fetus and its overall progress as it descends into the pelvis, which will assist with correctly performing the obstetric maneuvers required to relieve a shoulder dystocia, should one occur.[1][2]

Clinical Features

Retraction of the fetal head toward the perineum may be noted following the delivery of the fetal head. The "turtle sign" is usually a significant clinical sign of impending shoulder dystocia, but it is not necessarily diagnostic.[1] Although there is no consensus on the clinical criteria that are diagnostic of a shoulder dystocia, it is typically characterized by one of the following parameters:

• The delivering clinician is unable to deliver the fetal shoulders solely using gentle downward traction. 
• Additional obstetric maneuvers become necessary to deliver the neonate successfully.
• There is a documented head-to-body delivery interval of >1 minute.[1][2]

Evaluation

Pelvimetry and Pelvic Diameters 

Pelvimetry is the measured assessment of the dimensions of the pelvis; it is most often used to evaluate the risk of cephalopelvic disproportion in a pregnant individual. The 3 diameters primarily used in pelvimetry to assess the pelvic inlet are the anterior-posterior (A-P), the transverse, and the oblique diameters. Furthermore, the pelvic inlet's A-P diameter (ie, conjugate) can be measured in 3 different ways. By measuring the distance from 3 separate points along the pubic symphysis to the central point of the sacral promontory, various aspects of the A-P diameter, also known as A-P conjugates, can be assessed. These conjugates are the anatomical, diagonal, and obstetric conjugates.[15]

The anatomic conjugate is the distance between the sacral promontory and the superior margin of the pubic symphysis. Katanozaka et al. reported that parturients with an anatomic conjugate of <12 cm were more likely to have a cesarean delivery due to labor dystocia than those with larger conjugates.[16] The diagonal conjugate is the distance between the sacral promontory and the inferior margin of the pubic symphysis; it is the only conjugate that can be directly assessed on a physical examination. This is done by palpating the sacral promontory with the middle finger on a vaginal exam and then noting the distance from the fingertip to the point the examining hand contacts the pubic arch, which is approximately 12.5 cm on average.[15]

The obstetric (ie, true) conjugate is the distance between the sacral promontory and the widest portion of the pubic symphysis, located between the superior and inferior margins of the symphysis. The obstetric conjugate represents the smallest fixed distance through the pelvic inlet.[16] Although it cannot be measured directly on physical exam, it can be estimated by measuring the diagonal conjugate and subtracting 1 to 2 cm; the average obstetric conjugate measures approximately 10.5 cm. Some studies have demonstrated ultrasound imaging as an effective, safe, and simple way to assess the obstetric conjugate antenatally to identify patients who should be delivered by cesarean section due to a high risk for cephalopelvic disproportion.[16] Current guidelines, however, generally do not recommend making decisions regarding the mode of delivery based on pelvimetry assessments, as a Cochrane review found no evidence that pelvimetry imaging assessment improved outcomes.[16][17] 

Intrapartum Evaluation

Intrapartum conditions that may signal the possibility of impending shoulder dystocia include a prolonged second stage of labor, failure of the fetal head to descend, and need for the operative (ie, forceps or vacuum) vaginal delivery of the fetal head.[2] However, while it is essential to remain mindful of the risk factors for shoulder dystocia to prepare for delivery properly, attempts to predict shoulder dystocia based on risk factors have shown poor reliability and have a poor predictive value.[10] 

Therefore, shoulder dystocia remains a clinical diagnosis made after it occurs (eg, failure to deliver the fetal shoulders solely with gentle downward traction, additional obstetric maneuvers become necessary). Although professional societies have no agreed criteria to diagnose shoulder dystocia, maneuvers to manage a shoulder dystocia should be initiated if a shoulder dystocia is suspected due to characteristic signs (eg, turtle sign). Furthermore, suspicion of shoulder dystocia should be announced to the rest of the obstetrical team. Also, the delivery time of the fetal head should be noted so the team can track how much time has elapsed before shoulder delivery occurs. After delivery, careful clinical assessment is required for the infant (as well as the parturient) to look for potential traumatic injuries.[1]

Treatment / Management

Shoulder dystocia is an obstetric emergency requiring preparation and training for proper management by the delivering clinician and other obstetric team members. Not only does the infant need to be delivered quickly, but healthcare professionals must also use a careful and calm approach to mitigate the risk of injury to the parturient and the neonate.[1][2]

Once a shoulder dystocia is identified, it is important for the delivering clinician to clearly inform the other team members so they can immediately call for additional support (eg, neonatal and anesthesia clinicians), assist with any maneuvers, and begin proper documentation. The delivering clinician can then attempt specified obstetric maneuvers in a sequential order to relieve the shoulder dystocia. Maneuvers are typically divided into first-line, second-line, and heroic maneuvers.[2][18] Fundal pressure should be avoided during shoulder dystocia maneuvers. Not only does it not help with the delivery of the fetal shoulders, it may worsen the shoulder impaction and increase the risk of uterine rupture.[1] Furthermore, patients should be instructed to stop pushing while the maneuvers are being performed.[1]

First-Line Maneuvers

• The McRoberts maneuver: ACOG recommends this be the first maneuver performed; suprapubic pressure may be applied concurrently.[1] This maneuver involves two assistants, one holding each maternal leg, to hyperflex the patient's thighs onto the abdomen by bringing the patient's knees up toward her armpits. This increases the pelvic conjugates, resulting in a more open pelvic outlet.[1][8] It also causes a cephalad rotation of the pubic symphysis; this results in a significant increase in the angle of inclination (ie, the angle relative to the x-axis) between the superior border of the pubic symphysis and the superior border of the sacral promontory.[19] The angle between L5 and the sacral promontory flattens as the pelvis rotates with the McRoberts maneuver. As the sacral promontory flattens, the posterior shoulder of the fetus has additional space to move posteriorly and inferiorly into the true pelvis. Combined with the cephalad rotation of the pubic symphysis anteriorly, this allows the anterior shoulder to drop out from under the pubic symphysis, especially if suprapubic pressure is applied simultaneously, reducing stretch on the fetal brachial plexus, and facilitating delivery.[20] Flattening the sacral promontory also enables the force produced by spontaneous uterine contractions to become more effective by aligning the fetus with the maximal expulsive uterine force vectors.[21] Additionally, the position can significantly increase the intrauterine pressure generated during contractions and make maternal pushing efforts more effective; this is hypothesized to occur because the position brings the uterus closer to the diaphragm, which electromyographic studies have shown has a far more significant impact than the rectus muscles on generating increased intraabdominal and intrauterine pressure during Valsalva.[21] Therefore, when applied during contractions and maternal pushing efforts, the McRoberts maneuver significantly increases expulsive force, reducing mechanical resistance and facilitating delivery.[21]

• Suprapubic pressure: The goal of suprapubic pressure is to decrease the fetal bisacromial diameter by adducting the anterior fetal shoulder and dislodging the impacted shoulder. Continuous pressure is applied to the suprapubic area with the palm or fist in a downward and oblique direction toward the patient's right or left side, depending on which way the fetus is faced.[1][2] The delivering clinician should know the position of the fetus and communicate to the assistants the direction in which suprapubic pressure should be applied and who should apply it based on where each assistant is standing. For example, if the infant is in the right occiput anterior (ROA) position, facing down and toward the left maternal leg, then the left fetal shoulder is impacted under the pubic symphysis; the neonate should be rotated in a clockwise direction by the assistant holding the maternal right leg, who can more easily apply the suprapubic pressure by pushing down and slightly away from themselves, toward the maternal left hip. Conversely, if the neonate is in the left occiput anterior (LOA) position, the assistant holding the maternal left leg should apply pressure toward the maternal right hip and rotate the neonate in a counterclockwise direction.[1][2]
(B3)

Second-Line Maneuvers

• Delivery of the posterior arm: Some authors recommend this maneuver be used before other second-line maneuvers if the McRoberts maneuver and suprapubic pressure fail to result in delivery since some studies have suggested it may be most effective in this group.[1][6][1] To perform this maneuver, the obstetrical clinician slides a hand along the posterior fetal shoulder and arm, grasps the forearm or wrist, and sweeps the arm across the fetal chest to deliver the posterior arm. With the successful delivery of the posterior arm, the relevant transverse diameter changes from the bisacromial diameter (ie, shoulder-to-shoulder) to the acromial-axillary diameter (ie, axilla-to-shoulder), which can be up to 3 cm shorter. This dislodges the anterior fetal shoulder and allows for delivery. If the fetal forearm is not easily accessible (eg, when the fetal arm is extended), the following additional maneuvers can be attempted to gain access to the forearm or deliver the posterior fetal arm through alternative means:[2][6][2]
  • Fetal arm flexion: The delivering clinician slides a hand down the posterior fetal arm until the antecubital fossa is palpated; pressure can then be applied to the antecubital fossa, typically leading to flexion of the fetal arm and allowing access to the forearm.[2]
  • Menticoglou maneuver: The delivering clinician places their middle fingers under the posterior axilla and applies outward traction, which leads to delivery of the posterior shoulder, followed by delivery of the posterior arm.[2]
  • Posterior axilla sling traction: A suction catheter or a soft urinary catheter is positioned under the axilla of the posterior fetal arm and used as a sling. Traction is applied to the sling to deliver the posterior shoulder, followed by the arm. Alternatively, the sling can rotate the shoulders by applying lateral traction toward the baby's back. At the same time, the other hand is placed on the anterior shoulder, applying pressure toward the fetal chest.[1][2][8][22]
(B3)

• Rotational maneuvers (Rubin or Woods screw)
  • The Rubin maneuver: This is performed by the delivering clinician placing a hand into the vagina and applying pressure to the scapula of the posterior fetal shoulder towards the fetal chest. This will lead to the adduction of the fetal shoulder, allowing the anterior shoulder to rotate and deliver from behind the pubic bone where it is impacted.[1] 
  • The Woods screw maneuver: This is performed by placing one hand on the clavicular aspect of the posterior fetal shoulder and rotating the shoulder toward the fetal back. This allows the anterior fetal shoulder to descend while the rotation is occurring.[1]

• Gaskin maneuver: The patient is placed onto her hands and knees (ie, an "all fours" or "table-top" position); either gentle downward traction to the fetal shoulder closest to the maternal sacrum or upward traction to the fetal shoulder against the maternal symphysis pubis is then applied.[1][23] If this is unsuccessful, other maneuvers (eg, delivery of the posterior arm, rotational maneuvers) can also be attempted from this position. This maneuver should only be used in patients without epidural analgesia.[1]

• Routine episiotomy: Although some experts have recommended wide episiotomies be used before performing second-line maneuvers in all patients to gain more access, ACOG and the Royal College of Obstetricians and Gynecologists (RCOG) both recommend against universally performing an episiotomy in shoulder dystocia cases. This is because shoulder dystocia is, fundamentally, a boney obstruction, and incising the soft tissue of the perineum does not relieve this obstruction. ACOG and RCOG recommend considering an episiotomy only if the delivering clinician believes it will help them perform the second-line maneuvers better.[1][8]
(B3)

Heroic Measures: Extraordinary measures may need to be considered for severe or catastrophic shoulder dystocias that do not resolve with all other maneuvers. These maneuvers are associated with extremely high rates of fetal morbidity and mortality and high rates of significant morbidity for the parturient. They should only be used when multiple attempts at first- and second-line maneuvers have failed. The delivering clinician should have an assisting physician available and be aware of the time needed to prepare the operating room and administer general anesthesia.[1][2]

• Intentional clavicular fracture: The fetal clavicle is intentionally fractured by manually pulling the anterior clavicle outward. If successful, this will decrease the bisacromial diameter. This maneuver, however, is difficult to perform and may injure the underlying vascular and pulmonary structures.  

• Zavanelli maneuver: The fetal head is rotated to the occiput anterior position, flexed, and pushed back up the vagina and into the uterus, effectively reversing the cardinal movements of labor and relieving the compression on the umbilical cord. This fetus is held in place until emergent cesarean delivery is performed.  

• Abdominal rescue: If a cephalic replacement (ie, the Zavanelli maneuver) is unsuccessful, a low transverse hysterotomy is performed, and the fetal shoulder is dislodged and manually rotated through the transabdominal incision, followed by vaginal delivery.

• Symphysiotomy: A Foley catheter is placed with the patient in the lithotomy position. The urethra is retracted laterally with the Foley catheter. The pubic symphysis is then incised, and the anterior fibers and cartilage are divided in two. This is recommended only as a last resort when all the other measures have failed or in cases where immediate access to an operating room for Zavenelli or abdominal rescue is not available.[1][2][8]
(B3)

Documentation of Shoulder Dystocia

Documentation by a member of the obstetrical team of delivery times and sequence of events is vital not only to the delivering clinician but also to the patient and their future healthcare practitioners. Furthermore, following the delivery, proper documentation assists clinicians in reviewing what occurred during the delivery and counseling patients on subsequent pregnancy risks. It also provides any future obstetrical clinicians involved in that patient's care insight into the delivery events. Clinical information important to post-delivery documentation includes prepregnancy maternal weight, total maternal weight gain, estimated fetal weight, duration of active labor, duration of the second stage, and delivery time of the fetal head and body.[1][24](B2)

Differential Diagnosis

When retraction of the fetal head against the perineum is noted (ie, the "turtle sign"), differential diagnoses include a tight nuchal cord, fetal malposition, or normal delivery; these can be excluded with a brief examination for a nuchal cord following delivery of the head and when delivery of the anterior fetal shoulder is attempted.[1]

Prognosis

Once a shoulder dystocia is relieved, the fetal and maternal prognosis depends on the severity of complications that may develop. Maternal morbidities (eg, obstetric anal sphincter injury, bladder laceration) may result in long-term issues, such as fecal incontinence. The most common neonatal injuries, humerus and clavicle fractures and brachial plexus injuries, often resolve without permanent disability. However, approximately 10% may result in permanent neurologic injury. Simulation-based training has decreased the overall rate of shoulder dystocia-related complications.[1][2][25]

The risk of recurrent shoulder dystocia is estimated at ≥10%.[1][26] A comprehensive analysis of risk factors and the overall clinical picture should be reviewed and discussed with the patient in subsequent pregnancies. The risks and benefits of cesarean delivery should also be discussed with the patient since this is the only way to prevent recurrent shoulder dystocia confidently. According to ACOG, cesarean delivery should not be universally recommended to all patients with previous shoulder dystocia as it often does not reoccur; rather, decisions regarding the mode of delivery in future pregnancies should be individualized based on clinical factors and the patient's preferences.[1]

Complications

Maternal Complications

Shoulder dystocia can result in several maternal morbidities and are more frequent when "heroic maneuvers" are performed.[1]  Maternal complications include:

• Postpartum hemorrhage
• Obstetric lacerations, including urethral, bladder, and 3rd- and 4th-degree perineal lacerations with resulting urinary and fecal incontinence
• Femoral and lateral femoral cutaneous neuropathies from prolonged use of the McRoberts position secondary to nerve compression underneath the inguinal ligament
• Symphiseal separation secondary to hyperflexion of maternal legs
• Uterine rupture [1][2]

Neonatal Complications

Neonatal complications associated with shoulder dystocia include traumatic injuries, asphyxia, and death. Traumatic injuries, including brachial plexus injuries and clavicle and humerus fractures, occur at a total rate of 5.2% following delivery.[1] Injuries to the brachial plexus during birth can be categorized as upper lesions involving C5-C6 (ie, Erb palsy), lower lesions involving C8-T1 (ie, Klumpke palsy), or total plexopathies involving C5-T1. Horner syndrome and facial nerve injuries may accompany brachial nerve injuries.[27] Of these, C5-C6 injuries and clavicle fractures are significantly more common.[28] Children with upper lesions have the highest rates of full recovery, while those with total plexus palsies have the worst prognosis.[1] Brachial plexus injuries typically resolve in up to 20% of cases with conservative measures, but persistent deficits are noted in approximately 10% of neonates.[2] 

Fetal asphyxia, hypoxic encephalopathy, and death are fortunately much less common. They most often occur after an average head-to-body delivery time of 10.75 minutes or using >5 maneuvers but are less common when the dystocia is relieved after the first maneuver.[1] Head-to-body delivery time is not predictive of the severity of neonatal injury, however, as neonatal death has been seen to occur after intervals of <5 minutes.[1]