Identical twins physically connected in any way are said to be
conjoined.
Conjoined Twins are rare, occurring roughly once out of every 200,000 live births.
Most commonly found in India or Africa, it is believed that conjoined twins are
caused by genetic and environmental conditions which prevent the separation of
twins after the 13th day after fertilization. The survival rate of conjoined
twins is between 5% and 25%.
The first recorded conjoined twins were Mary and Eliza Chulkhurst, Pygopagus
twins born in 1100 in Kent, England.
There are no documented cases of conjoined triplets or quadruplets.
Types of Twinning
Conjoined twins are
classified by how they are joined -- the
Greek word,
pagos, means, 'that which is fixed'.
Craniopagus Twins: Twins of these type are joined at the
cranium (the top
of the
head or
skull).
Craniopagus Twinning occurs in only 2% of all
conjoined twin cases and is a very difficult type of twin to
separate.
Nida and
Hira Jamal of
Pakistan were of this type.
Pygopagus Twins: Twins of these type are positioned back-to-back
with a posterior connecton at the rump. Pygopagus Twinning occurs in roughly 20% of
all documented cases. Rosa and Josepha Blazek of what's now the Czech Republic were of
this type.
Thoraopagus Twins: These twins share part of the
chest wall and a
heart.
Thoraopagus Twinning is the most common of all
conjoined twins, occuring in roughly 35%
of all reported cases.
Cephalopagus Twins: These twins posess an anterior union of the
upper half of the body with two faces on opposite sides of a conjoined head
(the heart is sometimes involved). This condition is extremely rare. Twins
exhibiting both Thoraopagus and Cephalopagus properties are referred to as
Epholothoracopagus.
Parapagus (or Diprosopus) Twins: These twins posess a lateral union of
the lower half of the body, extending variable distances upward. This type of twinning
accounts for about 5% of all conjoined twins. The heart sometimes involved. Abigail
and Brittany Hensel of the United States, sharing a single
body from the chest down, are an example of this very rare type of twinning.
Ischiopagus Twins: These twins are joined at the base of the spine
(either above or below the coccyx) and make up roughly 6% of all conjoined twins.
Omphalopagus Twins: These are joined from the waist to the lower
breastbone and account for about 34% of all reported conjoined cases.
Parasitic Twins: This type of
conjunction involves
asymmetrical
conjoined twins, one
twin being smaller, less formed, and dependent upon the other.
Fetus in fetu: This type of twinning involves an imperfect fetus contained
completely within the body of its sibling.
Sources:
- TwinStuff
- http://www.twinstuff.com
- Types of Conjoined Twins
- http://zygote.swarthmore.edu/cleave4a.html