Distosia Bahu

01Pathophysiology

Shoulder dystocia occurs when the bisacromial diameter (the distance between the two fetal shoulders) is too large to pass through the anteroposterior diameter of the pelvic inlet. In the normal mechanism of labor, after the head delivers, the shoulders rotate into the anteroposterior plane and slide beneath the pubic symphysis. In shoulder dystocia, the anterior shoulder becomes impacted behind the pubic symphysis, halting further descent.

The head delivers normally because it can mold and flex to fit the pelvis, but the shoulders are rigid bony structures that cannot compress. Once the head is out and the anterior shoulder is trapped, the fetal chest is compressed within the birth canal. This compression of the umbilical cord between the fetal body and the pelvic wall creates a time-sensitive emergency because fetal oxygenation rapidly deteriorates. From this point, the obstetric team has a narrow window (generally cited as approximately 4 to 8 minutes) before the risk of irreversible hypoxic injury or death increases substantially.

The reason this condition preferentially injures the brachial plexus is mechanical: when excessive lateral traction is applied to the fetal head to free the trapped shoulder, the ipsilateral C5-C6 nerve roots are stretched. This results in Erb-Duchenne palsy, presenting as the classic "waiter's tip" posture (the arm is adducted, internally rotated, with the forearm pronated and wrist flexed). In more severe traction injuries, the lower brachial plexus (C8-T1) can also be damaged, producing Klumpke palsy (loss of hand grip, "claw hand"). Fractures of the clavicle or proximal humerus can also occur and are sometimes created intentionally as a last-resort maneuver to reduce the bisacromial diameter.

The pathophysiology also explains why gestational diabetes is the single most important maternal risk factor: chronic fetal hyperinsulinemia in response to maternal hyperglycemia causes disproportionate fat deposition around the fetal trunk and shoulders relative to the head, creating the size mismatch that leads to impaction.

02Classification and Clinical Manifestation

Shoulder dystocia does not have a formal staging or grading system like many other conditions. However, it is clinically useful to categorize it by severity and by the maneuvers required to achieve delivery:

Neonatal Complications

Maternal Complications

03Diagnostic Workup

Shoulder dystocia is an intrapartum clinical diagnosis. There is no prenatal test that definitively predicts it. This is a commonly tested concept: ultrasonographic estimation of fetal weight is unreliable for predicting shoulder dystocia, with a false positive rate exceeding 50% for macrosomia. The exam will test whether you understand that ultrasound alone is not an indication for cesarean delivery purely based on estimated fetal weight in the general population. Current guidelines recommend consideration of elective cesarean only when the estimated fetal weight exceeds 5,000 g in non-diabetic women, or exceeds 4,500 g in diabetic women.

The best initial "test" is simply recognizing the clinical scenario: the fetal head delivers, retracts (turtle sign), and the shoulders do not follow. There is no confirmatory test because the diagnosis is self-evident at the bedside. The exam will never ask "what is the best imaging study to confirm shoulder dystocia?" Instead, it will test your ability to recognize the clinical scenario and immediately proceed to management.

Postnatally, the diagnostic focus shifts to the neonate. The examiner should assess the Moro reflex (asymmetric Moro suggests clavicular fracture or brachial plexus injury), passive range of motion, grip strength, and overall neurologic status.

04Management and Treatment

The first and most important step upon recognizing shoulder dystocia is to call for help and avoid panic-driven excessive traction on the fetal head, which is the mechanism that causes brachial plexus injury. Gentle, controlled downward traction is acceptable, but forceful pulling is contraindicated.

McRoberts maneuver combined with suprapubic pressure is always the first intervention and resolves the majority of cases. This pair should be performed simultaneously. If this fails, the clinician proceeds to internal maneuvers: Rubin II, Wood corkscrew, or delivery of the posterior arm. There is no strict required order among the internal maneuvers; the choice depends on the position of the fetus and the clinician's experience.

A critical exam trap: fundal pressure is absolutely contraindicated in shoulder dystocia. Fundal pressure drives the anterior shoulder further into the pubic symphysis, worsening the impaction and increasing the risk of uterine rupture. The exam will often present a scenario where a nurse or assistant suggests fundal pressure, and the correct answer is always to refuse and apply suprapubic pressure instead. Suprapubic pressure is applied above the pubic bone, directed posteriorly toward the fetal face. Fundal pressure is applied at the top of the uterus and pushes the baby downward, which is the wrong vector entirely.

An episiotomy may be performed to provide more room for the clinician's hand during internal maneuvers, but it does not resolve shoulder dystocia by itself because the obstruction is bony, not soft tissue. The exam may test this distinction.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most commonly tested trap on this topic is the distinction between suprapubic and fundal pressure. The exam will frequently describe a scenario where the fetal head has delivered and retracted, then ask for the next best step. The wrong answer is fundal pressure; the correct answer is McRoberts maneuver with suprapubic pressure. Many students confuse these because both involve pressing on the maternal abdomen, but they are applied at completely different locations and produce opposite effects.

A second common trap involves risk factor recognition. The exam may present a diabetic mother with an estimated fetal weight of 4,600 g and ask whether elective cesarean should have been offered. The answer is yes for diabetic women with estimated fetal weight exceeding 4,500 g, and the threshold is higher (5,000 g) for non-diabetic women. Students who do not know these thresholds will get this question wrong.

Another tested concept is complication recognition. The exam may show a newborn after a difficult delivery with the arm hanging limply at the side, internally rotated, with an absent biceps reflex but intact hand grip. This is Erb palsy (C5-C6), not Klumpke palsy. The distinguishing feature is that hand function is preserved in Erb palsy and lost in Klumpke palsy.

Finally, understand that shoulder dystocia is unpredictable and unpreventable in many cases. Up to half of all cases occur in fetuses with normal estimated weight and without identifiable risk factors. The core competency being tested is not prediction but recognition and correct sequencing of maneuvers: McRoberts and suprapubic pressure first, internal maneuvers second, heroic interventions last. The exam rewards candidates who know the algorithm and can apply it under time pressure without resorting to harmful interventions like fundal pressure or excessive traction.