The incidences of shoulder dystocia as promulgated by the American Congress of Obstetricians and Gynecologists (ACOG) and the Royal College of Obstetricians and Gynaecologists (RCOG) are 0.6–1.4 % and between 0.58 % and 0.70 %, respectively [8, 9]. However, throughout the medical literature, the incidence of shoulder dystocia varies by more than a factor of 50, from 1 in 769 vaginal to 1 in 25 deliveries [8–12]. Rather than a true difference in incidence, the high variation is most directly attributable to inconsistencies in defining shoulder dystocia, as well as the at-risk population from which to calculate incidence. As shoulder dystocia is nonexistent prior to 32 weeks’ gestation and is a complication solely of cephalic vaginal delivery, the most appropriate denominator to consider for reporting shoulder dystocia incidence should exclude premature infants <32 week, as well as cesarean and breech vaginal deliveries. Based on a handful of prospective studies, it is likely that the incidence of shoulder dystocia with current mangement techniques among the at-risk population is between 3.35 % and 7 % [13–18].

Biomechanically, shoulder dystocia presents as a bony obstruction to delivery of the trunk resulting from misalignment of the fetus’ bisacromial width relative to the anteroposterior dimension of the maternal pelvic outlet (Fig. 11.2). Since the occurrence of shoulder impaction against either the pubic symphysis anteriorly or the sacral promontory posteriorly cannot be visualized, its diagnosis is subjective. Normal descent and delivery of the aftercoming shoulders is facilitated by spontaneous rotation of the bisacromial shoulder width to occupy the oblique diameter of the pelvic outlet, which occurs during spontaneous external rotation (restitution) of the fetal head outside the introitus. Failure of the shoulders to rotate to the normal oblique orientation may result from either insufficient passage of time between head and body rotation prior to the shoulders reaching the level of the pelvic outlet (as may occur during precipitous second stage or with operative intervention) or from relative head-to-body size asymmetry. The persistent anteroposterior position of the fetal shoulders within the pelvis generally is confirmed by digital palpation, although a retraction of the fetal head against the perineum (turtle sign) (Fig. 11.3), which occurs in a minority of cases, is suggestive. Definitive diagnosis is confirmed when an initial attempt with downward axial traction fails to deliver the trunk as it would under normal circumstances.

Another commonly accepted definition for shoulder dystocia is the need to use ancillary maneuvers beyond initial downward traction [8, 9, 19]. Such a retrospective definition is problematic for diagnostic use in the clinical setting since it presupposes shoulder dystocia recognition in order for ancillary maneuvers to have been used. It is well established that milder forms of shoulder dystocia are difficult to diagnose [20]. In a prospective study of more than 30,000 vaginal deliveries in which the degree of traction applied to the fetal head was estimated by the delivering clinician and correlated with neonatal outcome, two-thirds of the deliveries were estimated as involving higher than average degree of traction yet were not characterized as having shoulder dystocia [21]. The same study proved a direct correlation between the degree of traction used and the occurrence and severity of brachial plexus injury (Fig. 11.4), which was more strongly correlated with injury than the concomitant diagnosis of shoulder dystocia. Thus, to be considered as a complication of vaginal delivery with potential for fetal injury, a clinically more effective and utile definition for shoulder dystocia should be the clinician’s perceived need to increase traction above that level he/she would consider “usual.” Upon recognizing that “usual” traction is insufficient to deliver the shoulders, the obstetric provider is explicitly advised not to increase traction, despite the natural tendency to do so, but instead to initiate ancillary maneuvers designed to reduce the amount of further traction needed and thereby to minimize the risk of brachial plexus injury [13, 22].

Before discussing the management of shoulder dystocia once it occurs, a review of epidemiologic considerations is in order. Dynamic causes of shoulder dystocia wherein an otherwise normal-sized fetus’ shoulders become misaligned within the maternal pelvis owing to rapid descent will occur only intrapartum; it may be spontaneous and non-modifiable (as with precipitous second stage) or iatrogenic and thus modifiable (as with operative vaginal delivery).

Shoulder dystocia resulting from fetopelvic disproportion is most often the result of macrosomia and accelerated and asymmetric somatic fetal growth relative to head size [23, 24]. Less commonly, short maternal stature and/or contracted maternal pelvic dimensions can lead to obstructed shoulder delivery of a normally grown fetus.

The large-for-gestational-age (LGA) infant potentially is recognizable prior to the onset of labor. Antepartum risk factors for shoulder dystocia are the same as those for fetal macrosomia:

Signs of excessive gestational weight gain, abdominal circumference to head circumference ratio >1.04, and estimated fetal weight >90 % for gestational age in the third trimester should prompt diagnosis and treatment of possible impaired glucose tolerance, even among women with previously normal pregnancy glucose screen. Dietary counseling about elimination of high glycemic index foods should be provided, and such women should be encouraged to limit further weight gain, especially if already obese. The presence of signs of accelerated fetal growth among women demonstrating adequate glycemic control of pregestational or gestational diabetes on diet alone (mild gestational diabetes) favors empiric treatment with medication to prevent or reduce the risk of delivery complications, including shoulder dystocia (Fig. 11.5) [30, 31].

Whereas treatment of impaired glucose metabolism or mild diabetes when manifested as accelerated fetal growth can reduce the risk of shoulder dystocia, deliberate induction of labor for so-called impending macrosomia is associated with a higher incidence of shoulder dystocia compared to those managed expectantly [32]. Indeed, both induction of labor and use of epidural labor analgesia have been associated with an increased risk of shoulder dystocia, albeit a relatively weak correlation. Although unproven, this phenomenon may be the result of interfering with spontaneous fetal descent and oblique shoulder positioning prior to the onset of natural labor.

The strongest predictor of the occurrence of shoulder dystocia in an index delivery is the occurrence of shoulder dystocia at a prior delivery. Unless a fetal injury has occurred, it is possible that a parturient may not be aware that a prior delivery had been complicated by shoulder dystocia. When the history of a previous delivery is otherwise remarkable for a large-for-gestational-age infant (>90 %) or clavicle fracture, a review of delivery records for any difficulty with shoulder delivery is prudent. As with shoulder dystocia in general, the precise risk of recurrence varies with definition and population considered; however, the relative risk is above sixfold compared to women delivering an infant similar in size to the previous child who did not experience shoulder dystocia during the prior delivery [25].

Considerable debate persists concerning the counseling of pregnant women with either a history of shoulder dystocia and/or in whom the estimated fetal weight is in excess of 4 kg. Elective primary cesarean delivery prior to the onset of labor is not a cost-effective approach [33]. However, among the significantly smaller population of women whose prior deliveries complicated by shoulder dystocia resulted in injury or those with estimated fetal weight above 4.5–5 kg counseling about a prelabor, primary cesarean section should be offered. For all other women, a trial of labor remains the appropriate management strategy; anticipation of possible shoulder dystocia in such cases is best managed only by advance coordination of appropriate setting, staffing, and preparation of needed equipment once delivery is imminent.

Risk factors for shoulder dystocia correlate poorly with the actual occurrence of shoulder dystocia and do not appear to be cumulative. Even in the presence of risk factors – as well as in the total absence of risk factors – obstetric providers should be cognizant of the potential for occurrence of shoulder dystocia at all cephalic vaginal deliveries occurring beyond 32 weeks’ gestation. Given its emergent nature and lack of predictability in any given delivery, development of competence in the proper execution of shoulder dystocia maneuvers should occur before experiencing it in an actual delivery – ideally, with use of regularly repeated rehearsals and drills utilizing mechanical simulators (Fig. 11.6a, b) [34–40].

Compared to other procedures for which simulation-based training is utilized, shoulder dystocia simulation is the most evidence-based in its proven impact on actual clinical outcomes. The “Code D” protocol published by Inglis et al. is noteworthy for its initial steps (Fig. 11.7), which are required prior to the initiation of maneuvers: Upon recognition of difficulty delivering the shoulder, a “hands-off” waiting period lasting up to 1 min is observed [36]. This alone can reduce the likelihood and severity of shoulder dystocia [18, 41–44]. Help from additional personnel is summoned during this time, allowing for spontaneous external rotation (restitution) of the delivered head and additional time for the aftercoming body to rotate within the birth canal. If manual rotation of the shoulders into oblique is needed, pressure should be applied on the posterior aspect of the fetal shoulder (Fig. 11.8) so as to achieve relative adduction and decrease of the bisacromial diameter. On average, an enforced “hands-off” waiting period in the initial moments following recognition of obstructed shoulder delivery increases the subsequent head-to-body delivery interval by only 0.5 min, with minimal effect on fetal cord pH (Fig. 11.9) [36]. Thereafter, the orientation of the fetal shoulders is confirmed with direct digital palpation (Fig. 11.10) and, if needed, is manually adjusted to occupy the oblique dimension of the maternal pelvis. This maneuver, known as Rubin’s maneuver, is accomplished before reapplication of any traction to the fetal head [45]. Compared to starting with McRoberts maneuver, Rubin’s reduces the magnitude of subsequent traction and the resultant strain on the brachial plexus [46].