(CNN)Here's some background information about conjoined twins.
Conjoined twins are physically connected to one another at some point on their bodies.
About 70% of conjoined twins are female.
Conjoined twins are identical - they are the same sex.
The form of conjoined twinning refers to the point where the twins are joined. The top five, starting with the most frequent: thoracopagus (heart, liver, intestine), omphalopagus (liver, biliary tree, intestine), pygopagus (spine, rectum, genitourinary tract), ischiopagus (pelvis, liver, intestine, genitourinary tract), and craniopagus (brain, meninges). (Mayo Clinic)
Scientists believe that conjoined twins develop from a single fertilized egg that fails to separate completely as it divides.
The term "Siamese twins" originated with Eng and Chang Bunker, a set of conjoined twins who were born in Siam (now Thailand) in 1811. They lived to age 63 and appeared in traveling exhibitions. Chang and Eng both married and fathered a total of 21 children between them.
In 1955, neurosurgeon Dr. Harold Voris of Mercy Hospital in Chicago performed the first successful procedure separating conjoined twins.