The axillary arteries begin to form around the fourth week of embryo development in the early upper limb paddles (Mohammed, Khozin). Starting from a primitive capillary, a network enlarges into a central axial artery and later contributes to the three axillary artery segments (Carlson 2014). In the second segment, the thoracoacromial artery forms proximally to the lateral thoracic artery. It arises anteriorly at the superior border of the pectoralis minor (Grays). Variations of branching are thought to result from inherited variability or de novo mutations during embryo development. Overall, the causes are numerous, and our understanding of them is limited (Carlson 2014).