What it means ?
When and How this happens ?
What Causes this ?Two contradicting theories exist to explain the origins of conjoined twins.
The older theory is fission, in which the fertilized egg splits partially. Identical twins (mono-zygotic twins) occur when a single fertilized egg splits and develops into two individuals. Eight to 12 days after conception, the embryonic layers that will split to form mono-zygotic twins begin to develop into specific organs and structures. It's believed that when the embryo splits later than this — usually between 13 and 15 days after conception — separation stops before the process is complete, and the resulting twins are conjoined.
The second and more generally accepted theory is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. Which means two separate embryos may somehow fuse together in early development.
What might cause either scenario to occur is unknown.
What happens next?"Harsh truth"
Most conjoined twins die in the womb (stillborn) or soon after birth.
Conjoined twins must be delivered by cesarean section. About 40 to 60 percent of conjoined twins are stillborn. Of conjoined twins born alive, less than half survive long enough to be candidates for separation surgery.
Thoraco-omphalopagus (28% of cases):
Parasitic twins (10%):
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.
Syncephalus: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.
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.