First of all Siamese twins CANNOT be fraternal.
Siamese Twins are identical twins (monozygotic) that during development, due to a genetical anomaly have been conjoined.
The egg cell after being ferilised instead of splittong completey, as occurs normally in a case of monozygotic twins, remains partially connected.
Conjoined twins take the neme "Siamese" in memory of a famous pair of twins: Chang and Eng who originated from Siam. (present day Thialand).
Chang and Eng lived in the 19th century and at the time very little was known about conjoined twins. Chang and Eng repetidly inquired about the possibility of separation and consulted many doctors however they always received the same reply. The doctors refused to perform sugery because they were not sure if the two brothers shared the same venal system. After their death an autopsy was performed and it was then discovered that the twins could have been easilly separated because they only shared the same liver.
A case of Siamese twins occurs very rarely, at a rate of 1 birth for 100,000 children or 5% of monozygotic twins. Aproximately 75% of conjoined twins are stillborn or die within 24 hours of birth.
More males than females are conjoined in the uterus, however females are 3 times more likely to survive, resulting in a 70% rate of conjoined twins being girls.
Click here to have a full medical explanation of the types of conjoined twins.
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