Between 1964 and 2002, 26 pairs of conjoint twins were recorded at the Red Cross Hospital. The available radiographs and notes were reviewed, with specific attention to the incidence of spinal anomalies that result in scoliosis. Structural scoliosis was noted to occur only in the ischiopagus and pygopagus subsets, namely those joined by the pelvic outlet and the rump respectively. The abnormalities were largely those of failure of formation, with early onset of severe deformity. The hemi-vertebrae were often remote to the area of conjunction, mostly in the thoracic area. All six ischiopagi had vertebral abnormalities, with two of the four pygopagi demonstrating abnormalities. There were associated lower limb neurological abnormalities in the ischiopagi.

The association of conjoint twinning and vertebral anomaly is currently thought to be due to non-specific teratogenic insult with hypoxia. The fact that the ischiopagus and pygopagus are involved is important: these groups constitute up to 45% of survivors and are reported to have a longer life expectancy. Because they will later develop severe deformities, they need early active management.