Erb's palsy or shoulder dystocia is a condition that affects a bundle of five nerves known as the brachial plexus, which control muscles in the shoulder, arm, and hand. It typically occurs when excessive lateral traction is applied to the fetal neck region during delivery. This can cause one or more of the nerves to be torn, causing permanent paralysis of the arm. If the obstetrician recognizes the risk factors prior to the commencement of labor and/or properly manages the delivery when Shoulder Dystocia occurs, injury is avoided.

These risk factors for Shoulder Dystocia include:

• A previous delivery of a baby weighing over 4000 grams;
• A history of prior child who had Shoulder Dystocia;
• Maternal and/or gestational diabetes;
• Estimated Fetal Weight (EFW) over 4,000 grams (macrosomia);
• Maternal obesity;
• Gestational age over 41 weeks;
• A second stage of labor that lasts for more than two hours.

In addition, breech birth or other abnormal presentation of a fetus, while not a risk for shoulder dystocia, can result in brachial plexus injury.

Extra risk factors from prior pregnancies:

• a history of shoulder dystocia during a previous delivery
• a history of giving birth to large babies
• 8 years or more have passed since your last birth

Because of the loss of sensation from an epidural, a mother may not feel the pushing sensation, and the baby might descend in an awkward position. The use of tools, including forceps or vacuum, to help the baby descend, may increase the risk of shoulder dystocia as well. Studies also suggest an association between an increased risk for shoulder dystocia and the use of labor-inducing drugs.

Despite these risk factors, more women have uneventful birthing experiences than those whose labors are complicated by shoulder dystocia. In addition, shoulder dystocia can occur in the absence of these risk factors as well. A minority of obstetric brachial plexus impairments are not related to shoulder dystocia. Some other reasons for this disability include tumors in the baby's neck or the mother's uterus, viral disease or other abnormalities.