Introduction
Conjoined twins are extremely rare. They occur in approximately once every 200 000 births. As the term 'conjoined' suggests, these identical twins are joined together.
There is a very low chance of survival, with the survival rate capped at only 25%. 40 to 60% are delivered stillborn, meaning that they die even before leaving their mother's womb. 35% survive only one day.
There is a very low chance of survival, with the survival rate capped at only 25%. 40 to 60% are delivered stillborn, meaning that they die even before leaving their mother's womb. 35% survive only one day.
Conjoined twins are also known as Siamese twins.
How are they formed?
There are two contradicting theories.
1. Fission
When the fertilized egg splits into two, identical twins are formed. However, if the split occurs more than twelve days after conception, the embryos will not fully divide, resulting in conjoined twins.
2. Fusion
The fertilized egg initially splits into two, forming identical twins. However, while the twins lie side-by-side in the uterine wall, the embryos fuse together.
How is this possible? Well, a human embryo in its earliest stages consists of three layers of cells. These cells "seek out" cells of the same type and bond together to form individual organs. However, in the case of conjoined twins, when the two identical embryos lie side-by-side, very close to each other, the signals may get mixed. The cells of Twin A attach themselves to the cells of Twin B and although the cells are of the same type, they will end up sharing the same organ and become conjoined.
Here's an analogy to make it clearer. If the liver cells of Twin A attach themselves to the liver cells of Twin B, the twins will share the same liver and end up becoming conjoined!
Some Types of Conjoined Twins
Thoraco-omphalopagus: This is the most common type of conjoined twins.
Both bodies are fused from the upper chest to the lower chest. The twins usually share a heart and liver.
(Or part of the digestive system).
Thoracopagus: The twins’ bodies are fused from the upper thorax to the lower belly. In these cases, the heart is almost always conjoined.
Omphalopagus: In this case, the bodies are fused at the lower chest. The heart is never involved. However, the twins share a liver, digestive system, diaphragm and other organs.
Parasitic twins: Twins that are asymmetrically conjoined, resulting in one twin that is smaller, less formed, and dependent on the larger twin for survival.
Craniopagus: The twins have fused skulls but separate bodies. They can be conjoined at the back or front of the head, or the side of the head. However, they are not conjoined on the face or the base of the skull.
Cephalopagus: Two faces on opposite sides of a single, conjoined head. Only the upper portion of the bodies is conjoined. These twins generally cannot survive due to severe malformations of the brain.
Parapagus: The twins are fused side-by-side with a shared pelvis.
Credits
Diagrams of different types of conjoined twins: http://www3.telus.net/tyee/multiples/4conjoined.html
http://www.conjoinedtwins.med.sa/twins_type.php
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