Conjoined twins occur in roughly 1 in every 200 identical
twin pregnancies and are always identical. Actual numbers
for conjoined births vary from 1 in 20,000 to 1 in 100,000
pregnancies, and 40-60% are stillborn, with many others
dying within the first few days after birth. About 70% of
conjoined twins are female, the reason for which is
unknown.
Separation has been attempted on almost all conjoined
twins born since the 1950s with varying results, although
the first successful separation was performed in Basle,
Switzerland in 1689 on twin girls born joined by a
ligament at the sternum (xiphopagus). The first conjoined
twins to be successfully separated in "modern" times are
generally believed to be Catherine and Caroline Mouton of
Louisiana, born joined at the lower back (pygopagus)and
separated in 1953 at 8 days of age. Both survived the
operation, but Catherine committed suicide in 1985.
Formation
There are two theories of how conjoined
twins are formed. More widely accepted is the "fission
theory" which states that conjoined twins occur when a
fertilized ovum (egg) begins to split into identical
twins, but is somehow interrupted during the process and
develops into two partially formed individuals who are
stuck together.
The "fusion theory" has been around much longer, but only
with modern breakthroughs in understanding embryology has
it begun to look more probable. According to this model,
twins become conjoined after the fertilized ovum
intially splits into identical twins. While lying
side-by-side in the uterine wall, the two embryos become
fused together. A human embryo, in its earliest stages,
consists of three layers of cells. These cells "seek out"
cells of the same type and thus bond together to form
individual organs. When two newly-separated identical twin
embryos are lying in close proximity to one another,
sometimes signals get mixed and cells will attach to other
cells of the same type, but that belong to the other twin.
The easiest example of the fusion theory to understand is
the formation of craniopagus twins. One of the three
"sheets" of tissue that comprises the embryo will
ultimately "roll up" to become the neural tube - the
spinal cord and brain. Like rolling a piece of paper into
a tube and taping the edges, the edges of this
newly-formed tube will adhere together. Spina bifida, a
type of paralysis, is caused when these edges fail to join
properly. For this to happen, the cells at the edges of
the neural tube must be able to find and bond to one
another. However, when two neural tubes are present, the
cells at either end of the tubes might accidentally "find"
each other, and become stuck.
This "fusion theory" of conjoined twinning has been
espoused in recent years by such experts as Dr. Rowena
Spencer, who has studied over 1,300 sets of conjoined
twins in her 50-year career, and Armand Marie LeRoi,
author of the book Mutants which discusses the
embryological causes behind many of history's so-called
"freaks".
Not surprisingly, the media tries to dumb down the science
of conjoined twinning with such inaccurate phrases as
"fused since birth" or "joined at birth" - making it seem
as though conjoined twins are sort of forced into one body
during the birthing process. Perhaps a more scientifically
accurate description would be "joined since shortly after
conception."
Types of conjoined twins
Conjoined twins are categorized by a set of adjectives
ending with the suffix "-pagus" from the Greek word for
"fixed". Early teratologists such as Ambroise Pare and
Geoffrey St. Hillaire were among the first to identify and
name the various types of conjoined twins. Many actual
sets of twins do not fit perfectly into any of these
classifications, and the terms are often combined to
describe these twins.
Thoracopagus
(left).
Joined at the upper chest, from the clavicle
to the sternum, each with their own separate heads, arms
and legs. The heart is always involved in the conjoinment;
some thoracopagus twins have two separate hearts in a
single pericardium (heart sac), while others share a
single, malformed heart. Thus separation is extremely
risky and both twins often die, despite doctors' efforts.
In a very few cases, twins sharing a heart have survived
for several years. Ruthie and Verena Cady of Rhode Island
lived to the age of 7 and were healthy, active girls who
rode a tricycle, swam, did gymnastics and went to school.
Their shared heart had only 3 chambers. Thoracopagus twins
are the most common type, accounting for around 35% of all
cases.
Omphalopagus
(right).
Joined at the abdomen, from the sternum to
the groin and often sharing a liver and portions of the
digestive system. These twins have separate hearts, heads,
arms and legs. Separation is usually successful. Ronnie
and Donnie Galyon, born in 1951, are the only
non-separated omphalopagus twins in the world today. About
30% of cases.
Xiphopagus
(left).
Joined at the xiphoid process (part of the
sternum) and usually linked only by cartilage and soft
tissue. These twins share no vital organs but often have
conjoined livers. They are by far the easiest to separate.
Chang and Eng Bunker (1811-1874) were xiphopagus twins
with conjoined livers. Also called sternopagus. About 3%
of cases.
Ischiopagus
(right).
Joined at the ischium (front pelvis) and
lower spine (sacrum), with spines at 180-degree angles to
one another. These twins can have three legs (tripus) or
four legs (tetrapus). About 70% of ischiopagus twins have
four complete legs. In tripus cases, the third leg is a
fusion of two legs that is not controlled by either twin
and is therefore useless. Masha and Dasha Krivoshlyopova
of Russia (1950-2003?) were ischiopagus tripus twins,
their third leg having been removed when they were 16
years old. About 14% of cases.
Ischio-omphalopagus.
Combination of ischiopagus and
omphalopagus, with spines joined in a "Y"-shape. These
twins usually have three legs and a single set of
genitalia.
Parapagus.
Joined side-by-side at the torso, having
separate heads and arms and usually 3 legs. This is not a
totally accurate term, since many of these twins are
technically "xipho-thoraco-ischiopagus" or "xipho-omphalo-ischiopagus"
or other long, cumbersome appellations.
Dicephalus
(right).
A subset of parapagus. Twins share a body
from the neck or upper chest downward, having only two
legs and one set of reproductive organs. They can have
two, three or four arms. If separate hearts are present,
these twins have a good prognosis for long, healthy lives
if not separated. Abigail and Brittany Hensel (1990- ) are
healthy young girls who are dicephalus twins with two
hearts. They love to play sports and are not at all
limited by their conjoinment.
Craniopagus (left).
Joined by a portion of the
skull, with distinctly separate necks and bodies.
Separation is very risky since these twins can share parts
of the brain, as well as blood circulation. Craniopagus
twins are further classified by the portion of the skull
which is shared: vertical craniopagus - joined at the top
of the head with bodies at a 180-degree angle to one
another; occipital craniopagus - joined at the back of the
head; frontal craniopagus - joined at the forehead;
parietal craniopagus - joined at the side of the head.
Cephalopagus
(right).
Twins with conjoined necks and heads but
separate bodies. Due to malformations in the brain, these
twins are never viable. Those that are not stillborn die
within a few hours. Also called syncephalus or janiceps.
Cephalothoracopagus (left).
A combination of cephalopagus and thoracopagus. Twins with conjoined heads,
necks and chests, with separate arms and legs. These
usually share a heart as well as a brain and are also
non-viable. Also called epholothoracopagus,
prosopothoracopagus or craniothoracopagus.
Diprosopus.
Another subset of parapagus, describing
a single organism with two faces on a single head. It is
questionable whether some cases of diprosopus are even
caused by twinning; however, if diprosopic babies are a
form of conjoined twins, it would support a fission theory
of conjoined twinning. This condition is rare in humans
but relatively common in other animals such as sheep,
cattle and cats. Gould & Pyle refer to this as "disprosopus"
but I believe this to be a spelling error.
Pygopagus
(right).
Joined back-to-back at the pelvis and lower
spine, each with separate hearts, heads and limbs. Almost
all pygopagus twins are female; most male twins of this
type are stillborn. Historically, many pairs of conjoined
twins who live to adulthood have been pygopagus.
Separation is usually successful, although separating a
conjoined spinal cord can lead to paralysis below the
point of junction. Also called illeopagus.
Rachipagus
(left).
Joined back-to-back at any point above the
lumbar spine. Only one case of typical (non-parasitic) rachipagus twins has been recorded. These twins, reported
in 1960, were joined from the mid-spine to the top of the
head, with conjoined skulls. Rachipagus parasites are
slightly more common, with approximately 20 cases reported
(Spencer).
Parasitic twins.
Any of the above types can result
in a case of parasitic twins if one twin dies or receives
inadequate nutrition while in utero.
