Delivery of the Posterior Arm Reduces Shoulder Dimensions in Shoulder Dystocia

Roughly one in 200 births is complicated by shoulder dystocia—defined as a difficult shoulder delivery after delivering the head. The most common form is impaction of the anterior shoulder against the pubic symphysis, but impaction of the posterior shoulder against the sacral promontory or impaction of both shoulders can also occur. Most clinicians use the McRobert maneuver, suprapubic pressure, or the Woods maneuver to attempt to relieve the impaction before considering delivery of the posterior arm and shoulder—the “Barnum” maneuver. However, the Barnum maneuver might actually be helpful earlier if it provided a reduction in shoulder diameter and circumference compared with leaving both fetal arms by the sides. This study, which enrolled 33 term infants, was an attempt to compare a “one arm extended” technique with a “both arms by the sides” delivery to determine whether the former measure reduces shoulder diameter and circumference and whether it achieves a greater reduction in fetuses with relatively large shoulders. Within 72 hours of birth, bisacromial diameter (BAD) and axilloacromial diameter (AAD) were measured after tracing around the infant’s upper body while it was held motionless on a sheet; bisacromial circumference (BAC) and axilloacromial circumference (AAC) were measured directly. All measurements were made first with the newborn infant’s arms by its chest and then with one arm extended above the head. BAD values were comparable in male and female infants as were birth weights. Compared with the “both arms by the sides” technique, the “one arm extended” technique resulted in a reduction in BAD of 1 cm or more in approximately 90% of newborn infants, 1.5 cm or more in 72%, and a reduction of at least 2 cm in 44%. The mean ± standard deviation in BAD was 1.9 ± 0.69 cm. A similar mean reduction, 2.5 ± 1.2 cm, was seen in the AAC. Greater reductions were observed in newborn infants having greater shoulder diameters. The 2 diameters correlated with one another to a high degree. On average, the extent of reduction in the 2 diameters may be predicted from the following equation: Reduction (BAD − AAD) = 0.23 BAD −2.4 The difference between the 2 circumferences is accurately predicted by the following equation: Difference between BAC and AAC = 0.20 BAC −5.37 Compared with the “both arms by the sides” position, having one arm extended reduces shoulder diameter and circumference to a clinically useful degree. This is especially the case for heavier fetuses. Concern over manipulating the fetus may be overcome by adequate training using mannequins.

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