A brachial plexus injury is an often severe form of birth injury that is caused by physical trauma to the brachial plexus nerve cluster. These injuries are classified into three categories, including stretch, rupture and avulsion. These classifications describe the severity of the injury, with stretch being the most benign and avulsion being the most serious. A stretch brachial plexus injury occurs when the bundled nerve fibers at the base of the neck are stretched. These fibers run from the neck to the arm, and the stretching may range in severity from mild to serious.