They are further classified into eight basic types relative to their site(s) of union or pagus (Greek for joined): Omphalopagus (umbilicus/abdomen), thoracopagus (thorax/upper abdomen),
cephalopagus (maxillofacial), craniopagus (skull), ischiopagus (pelvis), rachipagus (spine) and pygopagus (sacrum).
Cephalopagus conjoined twins in a leopard cat (Prionailurus bengalensis).
The ventrally united twins may fuse at the oral end of the disc at the level of the septum transversum, the cardiogenic region, or the oropharyngeal membranes resulting in omphalopagus, thoracopagus, or
cephalopagus. They can also fuse at the caudal end of the disc at the site of the cloacal membrane resulting in the so-called ischiopagus twins.
Conjoined twins are classified according to the most prominent site of conjunction: thorax (thoracopagus); abdomen (omphalopagus); sacrum (pygopagus); pelvis (ischiopagus); skull (
cephalopagus), side by side (parapagus) and back (rachipagus).
Sites of fusion include hips (iliopagus twins), chest (thoracopagus twins), abdomen (omphalopagus twins) and head (
cephalopagus twins).
Thorax (thoracopagus 30-40%), abdomen (omphalopagus 25-30%), pyopagus (10-20%), pelvis (ischiopagus 20%), face (
cephalopagus), skull (craniopagus 2-16%), back (rachipagus).