In obstetrics, a shoulder presentation is a malpresentation at childbirth where the baby is in a transverse lie (its vertebral column is perpendicular to that of the mother), thus the leading part (the part that first enters the birth canal) is an arm, a shoulder, or the trunk. While a baby can be delivered vaginally when either the head or the feet/buttocks are the leading part, it usually cannot be expected to be delivered successfully with a shoulder presentation unless a cesarean section (C/S) is performed..