As is sometimes the case with genetic anomalies and mutations, sirenomelia or "mermaid syndrome" conjures certain fanciful images that have nothing to do with the traumatic effects associated with the actual medical anomaly.
Sirenomelia is so named because the birth defect involves the apparent fusing of the legs into a single lower limb, with the out-turned feet often resembling fish fins. The defect also commonly affects the kidneys, large intestines and genitalia. The condition sometimes affects the spine, brain and lungs as well.
According to Healthline.com, this rare condition occurs when a fetus develops only one umbilical artery (which pumps blood from the fetus to the placenta) and one umbilical vein (which returns blood to the fetus), while normal fetuses develop two umbilical arteries and one umbilical vein. The altered arrangement causes less blood and nutrition to reach the lower body, leading to the fused limbs and underdeveloped array of organs.