The incidence of shoulder dystocia is generally reported to be between 0.3 % and 1.5% with scattered reports listing values both higher and lower. The "true" incidence of shoulder dystocia, however, is very much dependent upon how it is defined, how it is reported, and the characteristics of the population being measured. For instance:

The Bulletin on Shoulder Dystocia by the American College of Obstetricians and Gynecologists (ACOG) lists the rate of shoulder dystocia as 1.4% of vaginal births.

The rate of shoulder dystocia in Great Britain reported by the Royal College of Obstetricians and Gynecologists as 0.6%

The rate of shoulder dystocia as reported by various authors is as follows:

Nocon (1993) 1.4

Baskett (1995) 0.6%

McFarland (1996) 0.7%

Bofill (1997) 3.3%

Gherman (1998) 1.4%

Stallings (2001) 1.7%

Foad (2008) 1.51

Chauhan (2014) 1.4

Tsur, in a 2011 study from Israel involving 240,000 deliveries, found that shoulder dystocia complicated 0.2% of all deliveries. Interestingly, in this study the rate of shoulder dystocia was seen to have declined from 0.4% in 1988 to 0.13% in 2009. The author feels that this was most likely due to an increasing rate of cesarean deliveries for suspected macrosomia.

Overland (2014), in one of the largest reports on the subject, reviewed data from 1,914,544 vaginal deliveries. The reported rate of shoulder dystocia in that group was 0.68%.

Parantainen (2014) evaluated 42,964 deliveries in Finland. He reported an incidence of shoulder dystocia of 0.42%.

Hansen, in a 2014 review article evaluating 28 published studies on the incidence of shoulder dystocia involving more than 16 million births, found a composite rate of shoulder dystocia of 0.4%.

The accuracy of reporting is an important variable in shoulder dystocia statistics. Many obstetricians are reluctant to write down in their delivery notes that a shoulder dystocia has occurred for fear that this will be a red flag attracting a malpractice suit should it later turn out that the baby suffered an injury. Some studies have shown that only 25% to 50% of shoulder dystocias -- as noted by objective observers in a delivery room -- are recorded by the delivering physician (Gonik, 1991).

How one defines shoulder dystocia, of course, affects its reported incidence. Some obstetricians will only report a delivery as involving shoulder dystocia if they had to employ specific maneuvers to deliver the baby's anterior shoulder. Others will record shoulder dystocia if there is any delay in the emergence of the shoulder following delivery of the head. In some cases a physician will only record shoulder dystocia when a fetal injury has occurred.

Finally, the characteristics of the delivery group being measured will affect statistics on shoulder dystocia. A study evaluating the incidence of shoulder dystocia in a population with a larger than average percentage of macrosomic neonates or of diabetic mothers will have a much higher reported incidence of shoulder dystocia than if the population were a general one containing a more representational sample of both small and large babies and the normal percentage of mothers with diabetes.

Several recent studies have shown a lower rate of shoulder dystocia than has been reported in the past. This is despite the fact that on average newborns are getting larger. For instance, the percentage of very large babies (>4000 gms) has gone up 300% between 1970 and 1988 (Johar, 1988). Moreover, the last several decades has seen a marked increase in average maternal weight, average maternal weight gain during pregnancy, and the number of diabetic women having babies. All of these factors should have lead to an increase the incidence of shoulder dystocia.

The likely answer to this paradox is several fold:

1. Physicians are now more aware of estimated fetal weight than in years past and are quicker to confirm this with ultrasound

2. Physicians are more reluctant to allow mothers with suspected macrosomic fetuses to have trials of labor but rather are recommending cesarean section for delivery.

3. Diabetic mothers with suspected macrosomic fetuses are especially being steered towards cesarean section for delivery.

One Step, Two Steps: An Interesting Theory

Iffy, in a 2015 article, claims that there has been an increased rate of shoulder dystocia and brachial plexus injury over the past several decades. He speculates that this increase is due to the advocacy of active management of delivery as proposed in the 1976 edition of Williams Obstetrics, whose major authors were Pritchard and McDonnell. This technique, called by some the “one step” technique, recommends attempting to deliver the infant’s shoulder immediately after the birth of the head. Locatelli (2011) has also discussed this issue.

Iffy notes that in prior editions of the Williams textbook (1961), whose major authors were Eastman and Hillman, it was expected that the shoulders would emerge in the contraction following delivery of the head—the “two-step” technique. Iffy denigrates the obstetrical experience of the authors of the newer edition, Prichard and McDonnell, saying that their expertise lay in cardiology, preeclampsia and basic sciences “rather than extensive experience in hands-on obstetrics.” Iffy also goes on to claim that prior to 2005 British obstetricians never adopted the “one step” policy and therefore had lower rates of brachial plexus injury—but that since they, too, have moved to the “one-step” technique they have seen an increase rate of BPI. Iffy concludes from the above that:

The rise of shoulder dystocia incidents since the introduction of active management suggest that elective use of traction is a major predisposing factor for both arrest of the shoulders and the deriving fetal injuries.”

However there are several aspects of Iffy’s contentions that deserve further scrutiny.

First of all, has there really been an increase in shoulder dystocia and brachial plexus injury—or has it just been reported more often due to medical-legal issues and better record keeping?

Second, Iffy’s arguments do not take into account the increasing percentage of macrosomic babies, the pronounced increase in maternal obesity, and the increasing use of epidural anesthesia as potential reasons for any increases in shoulder dystocia and brachial plexus injuries that might in fact have occurred.

Third, Iffy nowhere provides data to show that there is more traction involved in the “one-step” technique than the “two-step” technique.

Finally, Iffy’s argument only holds if clinician traction is the main cause of brachial plexus injuries when shoulder dystocias do occur. As will be shown below, while this has often been contended, it has never been proven—and there is much evidence against this hypothesis.

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Copyright © 2017 Henry Lerner