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Classification of conjoined twins
| Types | Definitions |
|---|---|
| Thoracopagus | Twins joined face-to-face from the upper thorax to the upper part of the abdomen; the heart is always involved |
| Omphalopagus | The fusion includes the umbilicus region, frequently at the lower thorax, but never the heart |
| Pyopagus | Twins fused dorsally sharing the perineal and sacrococcygeal areas; only one anus but two rectums |
| Ischiopagus | The union usually includes the lower abdomen and duplicated fused pelvic bones; external genitalia and anus are always involved |
| Craniopagus | Twins joined by the skull, sharing the meninges but rarely the brain surface; the face and trunk are not involved |
| Parapagus | Twins joined laterally, regularly share the pelvis. Varieties of parapagus conjoined twins are parapagus dithoracic (separated thoraces), parapagus dicephalus (one trunk two separate heads), and parapagus diprosopus (one trunk, one head, and two faces) |
| Cephalopagus | There are two faces and twins are joined from the top of the head to the umbilicus |
| Rachipagus | Twins fused dorsally; the defect may involve the dorsolumbar vertebral column and rarely the cervical vertebrae and the occipital bone |
Anomalies associated with conjoined twins
| Thoracic anomalies | Dextrocardia (in thoracopagus and dicephalic parapagus twins) |
| Gastrointestinal anomalies | Meckel’s diverticulum |
| Genitourinary anomalies | Duplex system |
| Musculoskeletal anomalies | Congenital dislocation of hip |