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

Thoracopagus conjoined twins

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

Omphalopagus conjoined twins

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

Xiphopagus conjoined twins

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

Ischiopagus conjoined twins

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

Dicephalus conjoined twins

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

Craniopagus conjoined twins

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

Cephalopagus conjoined twins

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

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 (see parasitic twins); 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.

Pygopagus

Pygopagus conjoined twins

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

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. See my Parasitic Twins Info Page for more.

Acardiac twins

An acardiac twin (acardiac meaning without a heart) is an incomplete fetus, usually consisting of a pair of legs, that shares the womb with a normal twin and is sustained by the twin's placenta. In other words, it is a parasitic twin that does not share a bodily connection with the normal twin.

Chronological History of Conjoined Twins
More Information on Conjoined Twins