Shoulder dystocia is not an uncommon obstetric complication, occurring in as many as 2 per 100 vaginal births. This obstetric emergency is associated with a number of adverse perinatal outcomes for both the mother and infant, the most serious of which remains neonatal brachial plexus injury. In a minority of cases in which there is prolonged impaction of the shoulders, birth asphyxia also may occur.
Obstetricians and other birth attendants must be fully prepared to effectively manage shoulder dystocia when it occurs. They also should understand the existing controversies regarding prevention and the pathogenesis of injuries associated with shoulder dystocia.
Shoulder dystocia generally is not a predictable event, which makes prevention extremely difficult. Because of the limited accuracy of ultrasound for estimating fetal size, the risk of shoulder dystocia and resulting injury must be fairly significant before prophylactic cesarean is considered as a preventive measure. There are, however, certain high-risk scenarios that call for consideration of prophylactic cesarean delivery.
For the past several decades, clinical research has focused on whether shoulder dystocia can be predicted and/or prevented. Overall, most analyses have shown us that shoulder dystocia can be only minimally predicted, at best, and that prevention of this complication as well as associated injury is far from a simple undertaking.
The leading risk factor for shoulder dystocia is excessive birth weight, yet not all cases of shoulder dystocia involve infants who weigh more than 4,500 g, or even more than 4,000 g. In fact, most shoulder dystocia cases actually occur when birth weights are less than 4,000 g – especially in nondiabetic pregnancies. (In diabetic pregnancies, most shoulder dystocias and brachial plexus injuries do occur in infants with birth weights greater than 4,000 g.)
The possibility that birth weight estimates may help us to predict and/or prevent shoulder dystocia also is hindered by the fact that it remains difficult to identify large babies prior to delivery. Clinical estimation of size and the use of ultrasound are the two most commonly employed techniques for estimating birth size, but both have limited accuracy and may either underestimate or overestimate fetal size. Most large babies, moreover, can successfully undergo vaginal birth without the complication of shoulder dystocia, let alone brachial plexus injury.
All told, these realities limit our ability to use estimated birth weight in selecting those pregnancies that might benefit from prophylactic cesarean delivery.
To consider prophylactic cesarean delivery, the level of risk for shoulder dystocia and resultant injury must be fairly significant. The following are two clinical scenarios in which the risk of complications reaches a level at which the option of prophylactic cesarean section (including informed consent) should be discussed with the mother:
▸ A pregnancy complicated by diabetes in which the estimated fetal weight is greater than or equal to 4,500 g. Some experts have suggested that this threshold should, in fact, be lower in diabetic pregnancies. However, utilization of a lower threshold (such as 4,000 g or 4,250 g) must come with the recognition that it will spur the use of more cesarean deliveries to prevent injury.
▸ A patient with a history of shoulder dystocia birth, particularly when the fetus is believed to be of similar or greater weight than the previously affected fetus.
Determining the recurrence risk of shoulder dystocia has proved difficult because, in most clinical series, a large proportion of women with a history of the complication will undergo scheduled cesarean delivery in their subsequent pregnancies. This bias toward operative delivery may lead to an underestimation of the true recurrence risk. Regardless of this potential estimation bias, unless the estimated fetal weight in the woman's current pregnancy is significantly less than that of the prior pregnancy, we should counsel women with prior shoulder dystocia and offer them prophylactic cesarean delivery.
With respect to the predictive value of labor abnormalities, studies have yielded mixed results. The bottom line is that labor abnormalities are not particularly useful in predicting shoulder dystocia – except for cases of a prolonged second stage of labor when there is suspicion of a large infant. This combination of factors should alert the physician to the potential for shoulder dystocia. Operative vaginal delivery should generally be avoided in this scenario, because delivery above an outlet station may further increase the risk of shoulder dystocia and resultant injury.
Management, Medicolegal Issues
As with any delivery, the goal of management should always be to deliver the infant as safely as possible, minimizing the risk of traumatic injury and birth asphyxia. In most cases of shoulder dystocia, the shoulders remain in an anterior-posterior position and fail to rotate. This creates the potential for brachial plexus injury as the nerves of the brachial plexus are stretched with the descent of the fetal head.