conjoined twins

or Siamese twins

Identical twins (see multiple birth) whose embryos did not separate completely. Conjoined twins are physically joined (typically along the trunk or at the front, side, or back of the head) and often share some organs. Symmetrical conjoined twins usually have no birth anomalies except at the areas of fusion and can sometimes be separated by surgery. In asymmetrical conjoined twins, one is fairly well developed, but the other is severely underdeveloped and dependent on the larger twin for nutrition. The underdeveloped twin may have to be surgically separated to save the larger twin. The term originally referred to Chang and Eng, born in 1811 in Siam, who were joined by a ligament from breastbone to navel. Widely exhibited, they married two sisters and fathered several children.

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