Shoulder dystocia and associated risk factors
Shoulder dystocia is an obstetric emergency with a potentially catastrophic outcome. Following the birth of the head, delivery of the shoulders and body is complicated by the impaction of the fetal shoulders in the maternal pelvis. Typically the term is used to describe deliveries in which manoeuvres other than gentle downward traction are required to complete the delivery of the anterior shoulder. The overall incidence of shoulder dystocia varies based on fetal weight, occurring in 0.6% to 1.4% of all infants with birthweights between 2500 g to 4000 g. In infants with a birthweight of 4000 g to 4500 g this rate increases to 5% to 9% (Baxley 2004). Incidence rates also vary depending on the criteria used for diagnosis.

Shoulder dystocia is associated with a high risk of physical and psychological complications for the mother and neonate. Common maternal complications include uterine rupture, postpartum hemorrhage (11%) and soft tissue damage to the cervix and vagina (3.8%) (Baxley 2004). Psychologically, mothers may experience postnatal depression, post‐traumatic stress syndrome and may have problems with maternal‐infant interaction (Coates 2004). Immediate fetal consequences include asphyxia and meconium aspiration. Following delivery, brachial plexus injuries are most commonly encountered occurring in 4% to 15% of infants (Baxley 2004). The brachial plexus is a major nerve network supplying the upper limb. It begins in the neck, extends into the axilla and can be injured by excessive stretching of the neck during birth. A large proportion of these injuries resolve within 6 to 12 months. Cases in which complete severance of nerve roots has occurred may require several stages of surgery to restore function, but less than 10% result in permanent injury. Bony injuries involving the clavicle and, less frequently, the humerus are also common.

Although attempts to correctly predict cases of shoulder dystocia have had limited success, several risk factors are associated with an increased rate of its occurrence. Higher birthweight is the common denominator connecting most current reports on maternal and fetal risk factors for shoulder dystocia. The related maternal risk factors include diabetes, obesity and multiparity.

Keller 1991 identified shoulder dystocia in 7% of pregnancies complicated by gestational diabetes. In a study of pregnancy complications and adverse perinatal outcomes associated with obesity, Codegren 2004 found that shoulder dystocia occurred three times more often in overweight women than in those of normal weight. Orskou 2003 found that women with parity greater than two had an increased risk of giving birth to infants weighing more than 4000 g (macrosomic) and hence were more likely to have adverse outcomes during birth including shoulder dystocia. There is also evidence that macrosomia associated with continued fetal growth in post‐term pregnancies poses a risk for shoulder dystocia (Baskett 1995).

A prior birth complicated by shoulder dystocia has also been identified as a risk factor in some studies (Baskett 1995; Ginsberg 2001; Smith 1994). For instance, Smith 1994 reported recurrent shoulder dystocia in five out of 42 women (12%) who had previously had pregnancies complicated by shoulder dystocia. However, Baskett 1995 reported a smaller recurrence rate of only 1% to 2%. In a retrospective study of 602 births complicated by shoulder dystocia, Ginsberg 2001 reported a recurrence rate of 16.7%. The wide variation in recurrence rates reported in these studies may be attributed to varied population demographics of the sample and variations in the clinical definition of shoulder dystocia leading to under‐ or over‐reporting of cases. Nevertheless, these studies show that women with a history of shoulder dystocia are at a higher risk of a subsequent dystocia than the general population.

Using the knowledge of these risk factors, efforts, such as caesarean and induction of labour, have been made to prevent shoulder dystocia in women at risk. Since the 1970s, certain pregnancy risk factors have been used to identify women in whom caesarean could potentially avoid shoulder dystocia. According to a study of the cost‐effectiveness of prophylactic caesarean for fetal macrosomia by Rouse and Owen (Rouse 1999), it would require over 100 caesareans to prevent a single permanent brachial plexus injury. Reflecting this view, a review of literature by The American College of Obstetricians and Gynecologists inferred that performing caesareans in all women suspected of carrying a macrosomic fetus is not appropriate. The same review proposed that planned caesarean delivery may be reasonable for the non‐diabetic with an estimated fetal weight exceeding 5000 g or the diabetic whose fetus weighs over 4500 g. Some authors, however, continue to support caesarean in all cases of fetal macrosomia diagnosed by ultrasound (O'Leary 1992).

The ability to detect macrosomic fetuses accurately using clinical estimation and ultrasound has been the topic of several studies (Delpapa 1991; Deter 1985; Levine 1992). A study by Gonen 1996 found that the predictive value of clinical estimation of fetal weight alone may be slightly higher than when combined with ultrasonography. The second part of this study determined the effect of prenatal diagnosis of macrosomia on the incidence of shoulder dystocia and birth trauma and hence the ability to prevent such occurrences. Although fetal macrosomia is an obvious predisposing factor for a delivery complicated by shoulder dystocia, most cases of shoulder dystocia and birth trauma occur in nonmacrosomic infants (Geary 1995). In support of this, the study concluded that the prenatal diagnosis of fetal macrosomia had little effect on the predictability and, consequently, the preventability of shoulder dystocia.

Prevention and management of shoulder dystocia
Induction of labour has been trialled as a preventative measure in women identified as having an increased risk of shoulder dystocia (Gonen 1997; Kjos 1993; Tey 1995). Two Cochrane reviews have assessed the role of induction of labour in preventing pregnancy complications including shoulder dystocia, Irion 1998 in cases of suspected fetal macrosomia and Boulvain 2001 in diabetic pregnant women. Both reviews concluded that there was insufficient evidence regarding the effect of inducing labour on preventing shoulder dystocia.

The efficacy of prophylactic treatment in women at risk of shoulder dystocia prior to the onset of labour remains controversial. The difficulty in correctly predicting women at risk of shoulder dystocia has contributed to this controversy. Due to this unpredictability, health practitioners caring for women in labour must be educated in the management of shoulder dystocia once it has occurred.

A key management strategy is the application of various manoeuvres. The American College of Obstetricians and Gynecologists has developed a shoulder dystocia drill to help better organise the emergency management of an impacted shoulder (ACOG 2000). The drill is a set of manoeuvres performed sequentially as needed to complete vaginal delivery. The manoeuvres are arranged from the simple, which require only movement of the mother, to the complex, requiring manipulation of the fetus.

Manoeuvres used late in the management of shoulder dystocia are complex, requiring direct manipulation of the fetus. These include the Woods manoeuvre, the Rubin manoeuvre, cleidotomy and the Zavenelli manoeuvre. The Woods corkscrew manoeuvre and the Rubin manoeuvre involve manual rotation and adduction of the fetal shoulders. Cleidotomy involves deliberate fracture of the clavicle to accomplish delivery but it is usually a difficult procedure, especially in large, mature fetuses. The Zavenelli manoeuvre remains the last resort in the management of shoulder dystocia and involves replacing the fetal head into the pelvis followed by a caesarean section.

Manoeuvres used in the early management of shoulder dystocia aim to relieve the dystocia by manipulating the fetal shoulders and increasing the functional size of the maternal pelvis. The pelvis is a rigid structure consisting of four bones united by cartilaginous joints and ligaments and forms the walls of the birth canal. In pregnancy, the joints and ligaments undergo temporary changes under the influence of the hormones relaxin, progesterone and oestrogen allowing some movement of the joints to facilitate birth (Miller 1997). Therefore, by manoeuvring the mother and placing external pressure on the pelvis it is possible to take advantage of this laxity and aid the passage of the fetus.

Suprapubic pressure, the McRoberts manoeuvre and the all‐fours position are manoeuvres implemented in the early management of shoulder dystocia. The application of suprapubic pressure aims to displace the anterior shoulder away from the pubic symphysis (the anterior point of union of the pelvic bones) to allow the fetus to enter the pelvis in an oblique diameter. This is advantageous as the entry of the birth canal (the pelvic inlet) is oval in shape with a wide transverse diameter. The McRoberts manoeuvre involves assisting the mother into an exaggerated knee‐chest position whilst lying flat (or slightly tilted upward) on her back. This does not change the actual dimensions of the pelvis but aids delivery by straightening the sacrum relative to the lumbar spine and rotating the pubic symphysis toward the mother's head allowing it to slide over the fetal shoulder (Gherman 2000). When there is a minor degree of shoulder dystocia, movement of the mother into an all‐fours position can dislodge the obstruction so the shoulders can negotiate the pelvis normally. This position acts as an 'upside‐down' McRoberts position and has the beneficial effects described above.

The mechanics of shoulder dystocia would suggest that the complication could be avoided by applying the manoeuvres outlined previously in a prophylactic manner rather than as treatment measures once shoulder dystocia has occurred. The use of the McRoberts manoeuvre or the adoption of the all‐fours position would maximise the functional size of the maternal pelvis thus minimising the chance of shoulder impaction. The application of suprapubic pressure from the onset of labour may prevent fixation of the descending shoulders under the pubic symphysis.

The efficacy, easy application and perceived safety of these manoeuvres may warrant their application as preventative measures in cases where the risk of shoulder dystocia seems high. There are, however, potential adverse effects related to the their use. These include symphyseal separation and sacroiliac dislocation resulting from excessive or prolonged maternal hip flexion (Heath 1999) and fetal injury resulting from the application of excessive pressure as it passes beneath the pubic symphysis (Hankins 1998). This review aims to determine the efficacy of such manoeuvres as intrapartum interventions for reducing the incidence of shoulder dystocia in women at increased risk. This will be determined not only by their ability to reduce the incidence of shoulder dystocia but also by any maternal or fetal morbidity arising as a consequence of their use.