The proximal portion of the right subclavian artery arises from the right side of the fourth aortic arch, while the distal portion arises from the right seventh intersegmental artery. The left subclavian artery arises from the left seventh intersegmental artery. Angiogenesis, driven by hypoxia and growth factors, leads bilaterally to the formation of the thyrocervical trunks from the subclavian arteries; this process is complete by the fourth week of gestation (Sadler 2019).