Facts About Multiples
An Encyclopedia of Multiple Birth Records

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How Are Conjoined Twins Formed?

Conjoined twins form when one sperm fertilizes one egg, but at some point during the stage where the single egg splits, the process stops, and the twins develop attached to one another. Identical twins will only be conjoined if they separate after the 12th day of conception. The twins will share a chorion, amnion and a placenta, making them monoamniotic/monochorionic and subject to TTTS (Twin to Twin Transfusion Syndrome) and cord entanglement (two reasons why many are miscarried). No one knows why conjoined twins occur, although genetic and environmental factors have been explored.

Conjoined twins are always of the same sex. Some types (dicephalus and ischiopagus for example) may only have one set of external genitalia (or none at all), leading many misinformed news people to refer to boy/girl conjoined twins or cases where one is a boy or girl and the other is unknown (in fact, the other twin will always be the same sex as the co-twin). DNA testing has to be done to determine the sex of conjoined twins who have no external genitalia. In some cases, after separation, the twin without the external genitalia has been turned into a member of the opposite sex (seen in at least 2 cases).

Some news articles, particularly those that have been translated into English may not accurately describe the joining of the twins. Every set will be joined in a fashion shown below, but may be described in articles in odd fashions (ie, "joined at the neck")

Numbers

Conjoined twins occur in approximately 1 in every 40,000 births, but only once in every 100,000 to 200,000 live births.

60% of conjoined twins are either stillborn or lost in utero, and 35% of twins that survive birth die within the first 24 hours (mostly because the most common locations of conjoinment involve important systems such as the heart). Out of 100 conceptions of conjoined twins, only 26 sets will survive birth and the first 24 hours.

Conjoined twins are most likely to be female - 71.62% are girls, while 28.38% are boys. This is because male twins are more likely to be miscarried (as are all male babies).

They are most likely to occur in India or Africa than in China or the US. (Although the rates in Vietnam have been much higher in recent years, possibly due to Agent Orange exposure).

The birth rate of conjoined twins in Africa is approximately 1 in every 14,000 births.

1 in every 400 identical twin pregnancies will be conjoined twins.

Of all monoamniotic, monochorionic monozygotic twins, 10% are conjoined.

There had only been 30 attempts in the world between 1928 and 1987 to separate craniopagus twins, only 26 of the 60 twins survived.

Between 1970 and 1977 the CDC registered 81 sets of conjoined twins born in North America.

Types of Conjoined Twins

Anterior is the front of the body, while posterior/dorsal is the back and lateral is the side. Ventral refers to the area of the abdomen. Keep in mind when you see photos, some types look very much alike, and the only way to tell what type they are is through x-rays and other medical tests.

Ventral union: twins united along the ventral aspect (eg, joined at the front)
	Cephalopagus - Anterior union of the upper half of the body with two faces on opposite sides of a conjoined head. Joined by the face, containing a single head and two bodies. The head has a single face with four ears, two on the back of the head. This type is extremely rare and non-viable. Also called Syncephalus, Janiceps or Janus twins. [Note - the picture is of cephalothoracopagus twins - cephalopagus twins do not have thoracic involvement, but I couldn't find a suitable illustration.]
	Cephalothoracopagus - Fused from the head to chest. There is only one brain and head (sometimes a rudimentary 2nd face), and they usually share a heart and have fused gastrointestinal tracts. They are non-viable. They are similar to Cephalopagus, but with fusion of the chest as well as the head, and are slightly more common than true cephalopagus twins. Also called Cephalothoracopagus Syncephalus, Epholothoracopagus or Craniothoracopagus.
	Thoracopagus - United face-to-face from the upper thorax down to the umbilicus, with heart involvement, 90% have a shared heart and 75% have extensively conjoined hearts that can't be surgically separated. Four arms, four legs, two pelvises. Accounts for about 35% of conjoined twins.
	Omphalopagus - Joined face-to-face primarily in the area of the umbilicus, and sometimes involving the lower thorax, but always preserving two distinct hearts. There is not even a cardiac vessel in common. Conjoined livers are very common. Two pelvis, four arms and four legs. Accounts for about 30% of conjoined twins.
	Thoraco-Omphalopagus: Combination of thoracopagus and omphalopagus, these twins will be attached from the upper chest to the umbilicus, and will usually share a heart. About 90% of thoraco-omphalopagus twins have a common pericardium and 75% have conjoined hearts. Also may have shared livers and gastrointestinal tracts. Four arms and four legs are present. Xipho-omphalopagus twins may look more like thoraco-omphalopagus but will not have heart involvement.
	Xiphopagus: Joined at the xiphoid process (the smallest of the three divisions of the sternum, below the gladiolus and manubrium). They are joined roughly from the navel to the lower breastbone. These twins rarely share vital organs aside from the liver. This is a subset of omphalopagus.
	Ischiopagus - United from the umbilicus to a large conjoined pelvis with two sacrum and two symphyses pubis. They are joined end-to-end with the spine in a straight line. Four arms, a variable number of legs, and in general, a single external genitalia and a single anus, but sometimes genitalia and located internally. Accounts for about 6% of all conjoined twins. Ischiopagus Dipus - Only two legs are present. This type may resemble some parapagus twins quite a bit, especially when standing. Least common (6.25 % of all ischiopagus twins) Ischiopagus Tripus - Three legs are present. Many have only one set of external genitalia (the other set sometimes presenting internally). The third leg can be fused (two legs fused into one), or vestigial (non-functioning). (About 31.25% of all ischiopagus twins) Ischiopagus Tetrapus/Quadripus- Four legs are present (About 68.75% of all ischiopagus twins) [Notes: My stats are of known ischiopagus types. Many are just listed as ischiopagus, and without pictures there was no way to tell if they were di, tri or tetrapus type. Pictured is a diagram of ischiopagus tetrapus/quadripus. Please note that some photos of ischiopagus twins can be easily confused with dicephalus/dithoracic parapagus twins, as they can look similar if sitting or standing.]
	Omphalo-Ischiopagus - A combination of omphalopagus and ischiopagus, these twins present joined as ischiopagus twins are, but face-to-face with a joined abdomen. Four arms, and a variable number of legs are present. The main indication here is a fusion of the pelvis, which you won't see in thoraco/omphalo/xipho-pagus twins. There has also been cases of thoraco-omphalo-ischiopagus and xipho-omphalo-ischiopagus twins where the union extends upwards to the chest.
Lateral union: twins joined side-by-side with shared umbilicus, abdomen, and pelvis
	Parapagus - Twins that share a conjoined pelvis, one symphysis pubis and one or two sacrums, united side by side. This type account for about 5% of all conjoined twins. It is possible for them to share a heart (if one has situs inversus of the heart), but rare. They may share the liver, kidneys and other organs. Parapagus twins will be of the tripus (3 leg) or tetrapus (4 leg) type. Parapagus twins will be tetrabrachius (4 arms). (Pictured is parapagus tripus tetrabrachius)
	Dithoracic Parapagus - When the union is limited to the abdomen and pelvis (does not involve the thorax). This is a subset of parapagus twins. They will be of the dipus type. They will likely have their own hearts, although some will have situs inversus (reversed organs) and could possibly share a heart as well. [Note: Many sets of dithoracic are referred to as dicephalus in articles] Dibrachius - Two arms are present (About 10% of all dicephalus/dithoracic twins) Tribrachius - Three arms are present (About 10% of all dicephalus/dithoracic twins) Tetrabrachius - Four arms are present (About 80% of all dicephalus/dithoracic twins) (Pictured is dithoracic dipus tetrabrachius)
	Dicephalic Parapagus (Dicephalus) - When the union includes the abdomen, pelvis and thorax. Dicephalus have one trunk with two heads. They are always of the dipus (two leg) type, and 3rd and 4th arms may present on their backs. This is a subset of parapagus twins. They may share a heart and other organs. The rarest type of parapagus twins is dicephalus dipus dibrachius. Dibrachius - Two arms are present (About 10% of all dicephalus/dithoracic twins) Tribrachius - Three arms are present (About 10% of all dicephalus/dithoracic twins) Tetrabrachius - Four arms are present (About 80% of all dicephalus/dithoracic twins) (Pictured is dicephalus dipus dibrachius)
	Diprosopic Parapagus - A single trunk and single head with two faces. Various degress of duplication of the face and brain can be seen. This is the most uncommon and rare type of conjoined twinning. Also called Monocephalic Diprosopos.
Dorsal union: twins joined at the dorsal aspect (at the back). There is no involvement of thorax and abdomen
	Craniopagus - United on any portion of the skull, except the face or foramen magnum (base of the skull). They share bones of the cranium, meninges, and occasionally brain surface. Two trunks, four arms and four legs. Accounts for about 2% of all conjoined twins. The twins can be joined end to end (vertical), at the back of the head (occipital), front of the head (frontal), side of the head (parietal/temporal). (Pictured is vertical craniopagus)
	Pygopagus (also seen as Pyopagus or Illeopagus) - They dorsally share the sacrococcygeal, perineal regions and occasionally the spinal cord. There is one anus, two rectums, four arms and four legs. Accounts for about 19% of all conjoined twins.
	Rachipagus - Twins fused dorsally above the sacrum and lumbar spine, involving different segments of the column. This type is extremely rare, with only one recorded set born in 1960, joined mid-spine to the top of the head with fused skills. There have been 20 cases of rachipagus parasitic twinning.

Only ischiopagus and parapagus/dicephalus twin types will list the number of arms and legs present, as these types can vary. All other conjoined types are tetrabrachius tetrapus (4 legs, 4 arms). A "normal" person is dibrachius dipus (2 arms, 2 legs).

In addition, I have found various other terms - synadelphus, describing twins with one body and 8 limbs, duplicata incompleta, describing any part which has not completely separated (ie, Dicephalus would be considered duplicata incompleta as there are 2 heads and only 1 body.

Conjoinment

These numbers are my own calculations from the World Conjoined Twins list circa 2001. It may not be an entirely accurate picture of conjoinment, due to many sets that can't be counted due to not knowing their conjoinment type, or mistyping them.

Separations (by Hospital)

18 - Children's Hospital of Philadelphia by 1997.

15+ - Children's Hospital of Boston had operated on 9 sets of conjoined twins by the mid-90's. (It is likely 15 or more now)
11 - Texas Children's Hospital had seen 11 sets of conjoined twins since 1964, and successfully separated 3 of those sets.
7 - Toronto's Hospital for Sick Children has separated 7 sets of conjoined twins and treated 10 sets as of December 2004.
6 - John's Hopkins has done 6 sets since 1982.
6 - Xinhua Hospital in China since 1982.
5 - Children's Hospital of Los Angeles has separated 5 sets of conjoined twins, the first on a parasitic set in 1966, and other sets in 1976, 1979, 1981 (neither surviving), and 2003. All sets were girls.
4 - Columbus Children's hospital has separated 4 sets (3 sets of girls, 1 set of boys)

The Red Cross Children's Hospital in Cape Town, South Africa, has treated 34 sets of conjoined twins (Note, treated not separated)
Great Ormond Street Children's Hospital has separated or led management on 19 cases of conjoined twins.

Separations (by Surgeon)

17 - Dr. James O'Neill, when at Philadelphia Children's Hospital, advised on 50 cases of conjoined twins and operated on 17 cases . He is now at Vanderbilt University in Nashville.
15 - Dr. Hendren of Boston Children's Hospital has operated on 15 sets since the late 60s.
12 - Professor Lewis Spitz of Great Ormond Street Hospital in London has done 12 separations.
9 - Dr. Abdullah Al-Rabeah and his team have separated 9 sets in Saudi Arabia.
7 - Dr. Robert Filler and Dr. Harold Hoffman operated on 7 cases in Toronto. Dr. Filler has also consulted on 5 other cases at various hospitals. Dr. Ted Votteler (ret.) operated on 7 sets at Children's Medical Center in Dallas, TX.
3 - Dr. Marion Walker in Utah, has separated 3 sets of craniopagus twins.

This list is somewhat outdated (by a few years)

Surviving Separation

The results may be slightly skewed to the both surviving end, because of many reports that said the twins were separated but never followed up on whether they both survived or not, so the number may be a bit lower.

The majority of separated twins will have some kind of disability, whether it be physical or mental. Omphalopagus and xiphopagus twins are most likely to have good health after separation. Many ischiopagus twins will have a physical disability due to the fact they often have only 2 or 3 legs between them. Craniopagus separated twins are most likely to suffer mental disabilities.

Fetus in Fetu and Parasitic Twins

Parasitic twins - A condition in which one fetus is poorly formed and attached to a complete fetus, completely dependant on it. This condition is called acardius-acephalus because there is no heart or brain in the parasitic twin, which is non-viable. Many cases present as extra hands, legs and other limbs, sometimes in strange places (attached to the back or head for example). The parasitic twin can vary from a single extra leg to a complete second body that is fully dependent on the first.

Fetus in Fetu - Situation in which an undeveloped fetus is encased in the body of its twin. Often, the fetus in fetu presents as a cyst.

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