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    Shoulder dystocia: What if you could see it coming?

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    SHOULDER DYSTOCIA: These two words cause consternation in every patient who has had one and in every physician or midwife who has had to deal with it. Occurring suddenly and unexpectedly, this emergency can disable an otherwise perfectly healthy baby for life. It is also a major cause of medical liability suits, currently constituting the second highest category of payouts in obstetrics, only behind asphyxia.

    What is especially disconcerting about shoulder dystocia is that until now obstetricians have had few means of predicting when it will occur or of effectively preventing it. Studies look at various schemes to accomplish the goal of predicting and preventing shoulder dystocia, but none has proved clinically feasible.1-3

    Difficulty of predicting shoulder dystocia

    Why has finding an effective method for the prediction and prevention of shoulder dystocia proven so elusive? There are three major reasons.

    1. Shoulder dystocia, while not rare, occurs only sporadically, in roughly 0.6% to 1.4% of vaginal deliveries.4,5 That has made it difficult to gain substantial experience and data on shoulder dystocia, or on the even more uncommon subset of it, deliveries where there is brachial plexus injury.

    2. Deliveries are not reproducible. One mother can deliver a large baby easily, while another of the same physical dimensions and with a baby of the same size may experience an extremely difficult shoulder dystocia delivery. Upon analysis, the main risk factors for shoulder dystocia—suspected macrosomia, gestational diabetes, and/or a history of previous shoulder dystocia—have a low sensitivity (they predict relatively few cases of shoulder dystocia) and an extremely high false-positive rate (they occur frequently in normal deliveries).1,3,6

    3. Traditionally, we have focused mainly on the size of the fetus by estimating birthweight.7,8 But the other component of shoulder dystocia—the size of the mother and her pelvis—has received scant attention.


    Figure 1
    To illustrate the problems of predicting which women will experience a shoulder dystocia delivery, let's examine the graph in Figure 1, which shows the distribution of birthweights from a group of babies with a normal vaginal delivery compared with a group who experienced shoulder dystocia that was associated with fetal injury. Figure 1 demonstrates that although babies who experience shoulder dystocia with injury are on average heavier than babies without shoulder dystocia, there is much overlap. Thus, any protocol seeking to avoid shoulder dystocia by performing cesarean deliveries based on estimated fetal weight would result in a large number of unnecessary cesarean sections per shoulder dystocia with injury prevented.9-11 The most favorable reported ratio shows that 443 extra C/S deliveries would have to be performed to prevent one permanent injury, even in a diabetic mother with an estimated fetal weight of over 4,500 g.10

    Another difficulty is communicating the concept of risk to patients. For some patients, any risk to the baby is unacceptable. That makes it challenging to help patients place in perspective the various risks: 1 in 150 (average risk of shoulder dystocia); 1 in 6,000 (average risk of permanent brachial plexus injury); and the risk inherent in a C/S delivery which is, after all, major surgery. Individual patients are often not particularly concerned about the burden to the health care system from unnecessary cesarean deliveries as measured by the "number needed to treat" in preventing a permanent brachial plexus injury; understandably, they are more concerned that the injury under discussion might be to their child.

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    Emily Hamilton, MD
    DR. HAMILTON is Associate Professor, Department of Obstetrics and Gynecology, McGill University
    Henry Lerner, MD
    DR. LERNER is Clinical Instructor in Obstetrics and Gynecology at Harvard Medical School

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