In the developing vascular system, the sinus venosus constitutes the inflow end of the developing heart, the anterior cardinal veins drain the cranial part of the embryo, and the posterior cardinal veins drain caudally to the heart. Unlike its anterior counterpart, the posterior cardinal veins almost entirely regress except for the iliac anastomosis, supracardinal, and subcardinal sections. The iliac anastomosis gives rise to the common, external, and internal iliac veins and the median sacral veins. Most anatomic variations of the external, internal, and/or common iliac veins result from deficits in embryogenesis (Mozes and Gloviczki 2014; Mehta et al. 2019).