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Erb’s palsy is caused by damage to certain nerves of the brachial plexus.  This is the underlying cause, but what leads to that damage is more varied. Injury to the nerves that run from the spinal cord, through the bones of the neck to the shoulder and finally down the arm, results in various degrees of weakness, paralysis, and loss of sensation.
These effects can be seen in any part of the arm, depending on which of the nerves were damaged. The overwhelming cause of this damage is related to how a child is born.
Damage During Childbirth
The number one cause of Erb’s palsy is damage to the nerves during childbirth. There are various ways in which this can happen, but essentially the problem occurs when the head and arm or shoulder are stretched apart. The nerves run from the spine to the neck, and then to the shoulder and down the arm, so pulling or stretching the body in this way also stretches the nerves.
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Mild stretching can stress the nerves, which is considered minor damage that will heal naturally and can be helped by physical therapy. A greater degree of stretching can damage the nerves more severely. They can actually become torn, or in the worst cases, ripped away from the spinal cord. Various conditions in and around the process of childbirth can lead to the stretching that causes mild to severe damage.
Manipulating the Baby During Childbirth
There are many different factors that can lead to nerve damage during childbirth, some involving the doctor or other medical professional manipulating the baby in such a way that the brachial plexus nerves are stretched and become damaged.
For instance, the head and neck may get pulled to the side as the shoulders are delivered from the birth canal. Another possibility is that the baby’s head gets pulled while the shoulders are still in the birth canal. The way in which the medical professional manages the instruments used in difficult deliveries, such as forceps or vacuum, can also cause stretching of the nerves.
When an infant is abnormally large, it can be a struggle to accomplish delivery through the birth canal. Sometimes the shoulders can become stuck, creating the need for emergent interventions. This can lead to the head, shoulder, or arm being pulled or tugged in such a way that the nerves get damaged. 
If a child is abnormally large, it can usually be detected in the last few weeks of pregnancy. At that point, a doctor must decide if a cesarean section should be done to avoid a difficult birth that can lead to Erb’s palsy and other complications.
A breech birth is another factor that increases the risk of Erb’s palsy. This is a birth in which the baby’s buttocks or feet emerge through the birth canal first, whereas, in a normal delivery, the head comes out first. Erb’s palsy can occur because, as the doctor pulls the baby’s feet out first, the arms are pulled upward over the head.
This can put stress on the brachial plexus and cause damage. As with a very large baby, a breech position is often detected in advance of labor and may lead to the decision for cesarean section.
Erb’s Palsy During Cesarean Section
A cesarean section (c-section), the surgical delivery of the baby through an incision in the abdomen, is usually done to avoid high-risk births. If a delivery is expected to be prolonged, or breech, or to have other significant risk factors, a doctor may decide to perform a cesarean section.
Erb’s palsy is generally not a risk of c-section deliveries, but it may happen in very rare cases. Only about one percent of cases of Erb’s palsy occurs during cesarean section births. 
Erb’s Palsy at Any Age
Erb’s palsy is overwhelmingly a condition of childbirth and infancy, but it is possible to sustain injuries to the brachial plexus at any age. Any kind of accident, violence, or other forms of injury that stretches those particular nerves can cause enough damage to trigger Erb’s palsy.
Some of the more common ways someone might injure nerves of the brachial plexus include contact in sports like football, trauma from motor vehicle collisions, bullet wounds, falls, and tumors putting pressure on the nerves.
Whatever the cause of Erb’s palsy, for infants especially, the damage is usually short-lived. The nerves are often able to heal naturally, given enough time, and most babies will grow into healthy children with no lasting effects. For those that have more severe injuries, the results may be permanent but are not usually severe.
- Brachial plexus injury in newborns: MedlinePlus Medical Encyclopedia. (n.d.). MedlinePlus - Health Information from the National Library of Medicine.
Retrieved from: https://medlineplus.gov/ency/article/001395.htm
- Erb’s palsy – Who is to blame and what will happen? (n.d.). PubMed Central (PMC). National Institutes of Health.
Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2724163/
- Brachial plexus injury - Symptoms and causes. (2018, December 5). Mayo Clinic.
Retrieved from: https://www.mayoclinic.org/diseases-conditions/brachial-plexus-injury/symptoms-causes/dxc-20127374