health

Conjoined twins are identical twins in which the two embryos fail to
separate completely before birth. It is thought that the condition results
from incomplete splitting after the twelfth day of fetal development. If
a fertilized human egg divides into two embryos before the twelfth day,
the identical twins will be born normally as two separate infants. The
longer the delay in the separation of the two embryos, the more complicated
the connections between the conjoined twins are likely to be.
Conjoined twins can be categorized in several different ways. One
distinction that is often made is between symmetrical (equal) conjoined
twins, in which both infants are well developed; and asymmetrical
(unequal) conjoined twins, in which an incomplete twin is joined to a
fully developed sibling. Conjoined twins are also classified according to
the points at which their bodies are joined. This system goes back to a
French biologist named Étienne Geoffroy Saint-Hillaire (1772–1844),
who based his terms on the formal medical words for parts of the body.
These categories are listed in the next section.