Conjoined twins are identical twins whose bodies are fused.

Incidence rate

The incidence ranges from one in 50,000 births to one in 100,000 births. About half are stillborn. They are more common in Southwest Asia and Africa.

The condition is more frequently found among female twins. The sex ratio is three to one.

Causes

Two contradicting theories explain conjoined twins. The older theory is fission, in which the fertilized egg partially splits. The second, and more generally accepted, is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find similar stem cells on the other twin and fuse the twins together.

Types

The most common type is thoracopagus - that is, two bodies joined from the upper thorax to the lower belly.

Thoracopagus

The heart is always shared in these cases.

Thoraco-omphalopagus

Two bodies are fused from the upper to lower chest regions. These twins usually share a heart, and may also share a liver and part of the digestive system.

Omphalopagus

Two bodies are fused at the lower chest. The heart is never shared in these cases, but the twins often share a liver, digestive system, diaphragm and other organs.

Parasitic twins

Twins who are asymmetrically conjoined, resulting in one twin that is small and less formed, and dependent on the larger twin for survival.

Craniopagus

The skulls are fused but the bodies are separate. These twins can be conjoined at the back, front or side of the head, but not at the face or the skull's base.