Neonatal Brachial Plexus Palsy and causation

A vaginal childbirth is the result of the internal (endogenous) expulsive forces of uterine contractions, usually supplemented by active maternal pushing. Depending on the clinical circumstances, additional external (exogenous) traction forces may be required from the birth attendant. This blend of internal and external forces varies from birth to birth. Women who have had a previous vaginal delivery, for example, may deliver successfully with uterine contractions alone and the role of the birth attendant may be simply to control and slow the delivery so that trauma to the maternal perineum from stretching by the fetal head.