Conjoined twins represent one of the rarest forms of congenital anomalies. Successful surgical separation of conjoined twins is even more unusual, since the majority of conjoined twins are incapable of being successfully separated. Conjoined twins are usually categorized by the point at which they are joined. There are five primary types of conjoined twins. Conjoined twins are always the same sex and same race.
Births are more likely to occur in India or Africa than in China or the United States.
The malformation occurs approximately two weeks after fertilization of the egg. The cause is unknown, but factors responsible for the failure of twins to separate may be influenced by genetics or the environment. Normally, the cells are supposed to divide. Conjoined twins appear to result from identical twins who develop with a single placenta from a single fertilized ovum. The earliest case of conjoined twins on record appears to be that of the Biddenden Maids who were born in England in 1100.