Antero-posterior radiographic view, showing missing ribs, absent lumbosacral vertebrae, hypoplastic pelvis and "frog-like" position of the lower extremities The condition arises from some factor or set of factors present during approximately the 3rd week to 7th week of fetal development. Formation of the sacrum/lower back and corresponding nervous system is usually nearing completion by the 4th week of development. Due to abnormal gastrulation, the mesoderm migration is disturbed. This disturbance results in symptoms varying from minor lesions of the lower vertebrae to more severe symptoms such as complete fusion of the lower limbs. While the exact cause is unknown, it has been speculated that the condition has a combination of environmental and genetic causes, and that various types of the condition may have differing causes. Sacral agenesis<unk>  (a type of<unk>  involving agenesis of the lumbar spine, sacrum, and coccyx, and hypoplasia of the lower extremities) is a well-established congenital anomaly associated with maternal diabetes mellitus. Other causes are presumably involved, as demonstrated by the rare overall incidence of<unk>  (1:60,000) compared to diabetes; however, the condition does have a greatly increased incidence among infants born to mothers with diabetes, estimated at 1 in 350 newborns of mothers with diabetes. The dominant inherited sacral agenesis (also referred to as Currarino<unk>  is very often correlated with a mutation in the Hb9 (also called HlxB9) gene (shown by Sally Ann Lynch, 1995, Nature Genetics). It was previously thought to be related to sirenomelia ("Mermaid<unk> ") but has now been determined not to be related to this condition.
Caudal regression syndrome
Spina bifida
Cervical agenesis
Cranioschisis
Cernunnos deficiency
Exencephaly
Anencephaly
Gonadal dysgenesis