July 1996
. Twins can be conjoined at the:
Abdomen (omphalopagus).
Chest (thoracopagus).
Top of head down to the belly button, facing each other (cephalopagus).
Head only (craniopagus).
Pelvis, facing each other (ischiopagus).
Pelvis, side-to-side (parapagus).
Rump-to-rump (pygopagus).
Vertebral column (rachipagus).
Generally, parapagus are conjoined at the upper chest. Parapagus, united laterally, always share a conjoined pelvis with one or two sacrums and one symphysis pubis. Dithoracic parapagus is when the two chests are separated, and the fusion is confined to the pelvis and abdomen. Dicephalic parapagus is if there is the union of the entire trunk but not the heads. The heart, liver, and diaphragm are fused, but there is a duplication of the respiratory tract and upper digestive tract; the viscera organs are fused. There are two arms, two legs, and two complete vertebral column and spinal cord. The number of limbs varies from 4 to 7, rarely with four legs. Generally, each lung is present in a separate lung cavity. The fusion of lungs is very rare. The alignment of the conjoined pelvis is diagnostic-one complete pelvic ring, with a single anterior pubic symphysis, and with two laterally fused sacral bones, and predominantly only one rectum. Ischiopagi are united ventrally extending from the umbilicus down to a sizeable conjoined pelvis with two symphyses pubis and two sacrum. Craniopagus can be united at any portion of the skull except at the face and the foramen magnum. Pygopagus varieties are joined dorsally; sharing the sacrococcygeal and perineal regions, sometimes even involving the spinal cord. Rachipagus twins are united dorsally above the sacrum. The union may also include the occiput. The cephalopagus varients are fused from the umbilicus to the top of the head. The pelvis and lower abdomen are usually not fused. Thoracopagus are united face-to-face from the upper thorax down till the umbilicus. Omphalopagus are primarily United at the umbilical region aligned face to face. The pelvis is not united. The pure parapagus is two heads, two hands, two legs, two hearts and two pairs of lungs.
Conjoined twins are classified on the basis of the union's site, with the suffix pagus meaning fixed or fastened. The twins can have four (tetrapus), three (tripus), or two (bipus) legs.
Cephalopagus: The twins often have a fused thorax in addition to a fused head. The single fused head may have two faces (janiceps)
Cephalothoracopagus twinning is characterized by the anterior union of the upper half of the body, with two faces angulated variably on a conjoined head. The anomaly is occasionally known as janiceps, named after the two-faced Roman god Janus. The prognosis is extremely poor because surgical separation is not an option, in that only a single brain and a single heart are present and the gastrointestinal (GI) tracts are fused.
Craniopagus: The conjoined twins share the skull, meninges, and venous sinuses
Ischiopagus: The twins may lie face to face or end to end
Pygopagus: The twins are joined dorsally, sharing the sacrococcygeal and perineal regions
Rachipagus: The twins generally have vertebral anomalies and neural tube defects.
Thoracopagus: The twins lie face to face and share the sternum, diaphragm, upper abdomen wall, and liver and have an exomphalos