Until recently, Erb's palsy has been attributed to damage during birth caused by the birth attendant stretching the nerves of the brachial plexus. A review by Sandmire and DeMott concludes that the condition is a consequence of propulsive forces during birth, and that birth attendants have been unjustly blamed for outcomes over which they have no control.
The authors propose that the damage to the brachial plexus takes place between contractions during the second stage of labor, particularly if the leading shoulder does not move. Contractions plus maternal pushing provide strong propulsive forces that are followed by significant retractive forces. Propulsive and retractive forces are each capable of damaging the nerves, but significant damage is most likely to occur when these forces are combined. About one half of cases of Erb's palsy occur in deliveries without shoulder dystocia. The infants in these cases are smaller than infants in cases associated with shoulder dystocia, and the palsy is more likely to be permanent. More evidence for the importance of the second stage of labor comes from studies showing that instances of precipitate second-stage labor plus shoulder dystocia have a frequency of Erb's palsy in the infant that is 4.7 times greater than the rate occurring when the second stage is of normal length. In these studies, the independent variable is the length of the second stage. Erb's palsy has also been reported following completely unattended deliveries and cesarean deliveries. The incidence of the condition is independent of the level of expertise of the delivering physician.
Although frequently occurring concurrently, Erb's palsy and shoulder dystocia are now believed to be random and unpredictable events. Shoulder dystocia is more common in larger infants and is primarily related to fetal malposition and not to fetopelvic disproportion as previously believed. The recurrence rate is about 14 percent in subsequent deliveries.
The authors conclude that the available evidence overwhelmingly supports the view that Erb's palsy is seldom, if ever, due to excessive traction on the fetal head and neck during birth. They call for wider acceptance of the notion that the condition results from propulsive forces over which the birth attendant has no control.