Factfile: Conjoined twins

Conjoined twins have existed for centuries, with the first recorded pair being born in Armenia in 945.

The best known conjoined twins were Eng and Chang Bunker, born in Siam in 1811 -- hence the phrase Siamese Twins.

They led a full life, marrying the sisters Sallie and Adelaide Yates, and fathering 21 children between them.

The first successful operation to separate a pair of conjoined twins occurred as early as 1689 by German physician G Konig.

About 200 surgical separations have taken place, the bulk of which have happened since 1950.

More than 30 separate types of conjoined twins have existed, classified by the point at which they are joined. The most common is the thoracopagus type which in a joining of the upper half of the trunk and involves the sharing of the heart.

As many as one in every 40,000 births could be conjoined but only one in every 200,000 live births are conjoined.

The overall survival rate of conjoined twins is between five percent and 25 percent with about 75 percent of the surgical operations since 1950 resulting in one or both of the twins surviving.

Conjoined twins originate from a single fertilised egg dictating that they are always the same sex and race.

Females are more susceptible to be conjoined than men at a ratio of 3:1.

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