The ultimate cause of<unk>  is a subject of debate. The first hypothesis of its origin was developed in 1927 and proposed that a lack of blood supply to the lower limbs during their development is responsible for the defect. This "vascular steal" hypothesis was developed in response to the observed absence or severe underdevelopment of the aorta below the umbilical artery, which "steals" the blood supply from the lower limbs. Other hypotheses involve an insult to the embryo between 28–32 days affecting the caudal mesoderm, a teratogen exposure affecting the neural tube during neurulation, and a defect in the twinning process that either stops the process of caudal differentiation or generates a second primitive streak. Maternal diabetes mellitus has been associated with caudal regression syndrome and<unk> , although a few sources question this association. Prenatal cocaine exposure has also been suggested as an association with<unk> . Genetics In animal models, several genes have been found to cause or be associated with<unk> . The srn (siren) gene is observed to cause hindlimb fusion in homozygous mice. Mice with knockouts or mutations in both tsg1 and bmp7 will also have hindlimb fusion.
Sirenomelia
Rhizomelia
Mesomelia
Dolichostenomelia
Phocomelia
Meromelia
Loxoscelism
Adiposogenital dystrophy