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Types of conjoined twins
Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:
- thoraco-omphalopagus (28% of cases): Two bodies fused from the upper chest to the lower chest. These twins usually share a heart, and may also share the liver or part of the digestive system.
- thoracopagus (18.5%): Two bodies fused from the upper thorax to lower belly. The heart is always involved in these cases.
- omphalopagus (10%): Two bodies fused at the lower chest. Unlike thoracopagus, the heart is never involved in these cases; however, the twins often share a liver, digestive system, diaphragm and other organs.
- parasitic twins (10%): Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin for survival.
- craniopagus (6%): Fused skulls, but separate bodies. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.
Other less-common types of conjoined twins include:
- cephalopagus: Two faces on opposite sides of a single, conjoined head; the upper portion of the body is fused while the bottom portions are separate. These twins generally cannot survive due to severe malformations of the brain. Also known as janiceps (after the two-faced god Janus) or syncephalus.
- synecephalus: One head with a single face but four ears, and two bodies.
- cephalothoracopagus: Bodies fused in the head and thorax. In this type of twins, there are two faces facing in opposite directions, or sometimes a single face and an enlarged skull.
- Xiphopagus: Two bodies fused in the xiphoid cartilage, which is approximately from the navel to the lower breastbone. These twins almost never share any vital organs, with the exception of the liver. A famous example is Chang and Eng Bunker.
- Ischiopagus: Fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle. These twins have four arms; two, three or four legs; and typically one external set of genitalia and anus.
- omphalo-ischiopagus: Fused in a similar fashion as ischiopagus twins, but facing each other with a joined abdomen akin to omphalopagus. These twins have four arms, and two, three, or four legs.
- parapagus: Fused side-by-side with a shared pelvis. Twins that are dithoracic parapagus are fused at the abdomen and pelvis, but not the thorax. Twins that are diprosopic parapagus have one trunk and one head with two faces. Twins that are dicephalic parapagus have polycephaly, and two (dibrachius), three (tribrachius), or four (tetrabrachius) arms.
- craniopagus parasiticus: Like craniopagus, but with a second bodiless head attached to the dominant head.
- pygopagus (Iliopagus): Two bodies joined back-to-back at the buttocks.