How Does This Kind Of Twinning Occur?

You've seen conjoined twins on the news, and have probably wondered: How did those babies come to be that way? Here are some facts for you.
Since 1978, surgeons at Columbus Children's Hospital have successfully separated four sets of conjoined twins. They provided these facts about this very unique kind of twinning.

  • 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 occur in approximately one in 50,000 births, and the majority do not survive (they are stillborn).

  • Conjoined twins are always the same sex and same race.

  • The condition occurs more often in females than males at a ratio of 3:1.

  • Births are more likely to occur in India or Africa than in China or the United States. (In Africa, the reported frequency is one in 14,000 births, suggesting a higher incidence in blacks.)

  • 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. They were joined at the hips and lived to age 34, when one died and was followed six hours later by her sister.

  • History's most notable conjoined twins were Chang and Eng Bunker, born in Thailand (formerly Siam -- hence the origination of the name "Siamese Twins") in 1811. They were joined at the chest and traveled with Barnum and Bailey's Circus. They died at age 62, within three hours of each other.

  • There are no documented cases of conjoined triplets or quadruplets.

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