It depends on at which point during development did the fertilized egg partially separate into two embryos and also at which point does the partial separation stops. The process of partial separation usually occurs within the first weeks following fertilization. Most common are Thoracopagus twins that share a heart (up to 40%), followed by omphalopagus (30%) that share a liver and GI functions, the rarest (2%) are craniophagus that share a head.
