Table 1 Classification of conjoined twins based on the theoretical site of union [9].
From: Prenatal diagnosis of pygopagus tetrapus parasitic twin: case report
Embryonic aspect | Type | Incidence | Extent of union |
|---|---|---|---|
Ventral (87 %) | Â | Â | Â |
   Rostral (48 %) | Cephalopagus | 11 % | Top of head to umbilicus |
| Â | Thoracopagus | 19 % | Thorax, upper abdomen, conjoined heart |
| Â | Omphalopagus | 18 % | Thorax, upper abdomen, separate hearts |
   Caudal (11 %) | Ischiopagus | 11 % | Lower abdomen, genitourinary tract |
   Lateral (28 %) | Parapagus | 28 % | Pelvis, variable trunk |
|  |    Dicephalus 2 heads |  |  |
|  |    Diprosopus 2 faces |  |  |
Dorsal (13 %) | Craniopagus | 5 % | Cranial vault |
| Â | Pygopagus | 6 % | Sacrum |
| Â | Rachipagus | 2 % | Vertebral column (midportion) |