Conjoined Twins

Updated: Aug 30, 2022
Author: Khalid Kamal, MD, MBBS, FAAP, FCPS, MCPS; Chief Editor: Robert K Minkes, MD, PhD, MS 



Conjoined twinning is one of the most fascinating human malformations and has also been reported in other animals—mammals, fishes, birds, reptiles, and amphibians.[1, 2, 3, 4]  The term Siamese twins, once commonly used for conjoined twins, has fallen out of favor because it seems to convey that these individuals are circus freaks or monsters. Conjoined triplets have been described but are quite rare.[5]

The major types of conjoined twins were described in 1573 by the French renaissance surgeon Ambroise Pare. Approximately 75% of conjoined twins are female, and 70% are fused at the thorax (thoracopagus) or abdomen (omphalopagus). The union can be in the frontal, transverse, or sagittal plane. The following two photographs show union in a transverse plane in the lower body area, so that the twins face each other.

Union in a transverse plane in the lower body area Union in a transverse plane in the lower body area, so that the twins face each other.
Union in a transverse plane in the lower body area Union in a transverse plane in the lower body area, so that the twins face each other.

The two main categories of conjoined twinning are as follows:

  • Symmetrical or equal conjoined twins (ie, two well-developed babies)
  • Asymmetrical or unequal conjoined twins (ie, a small part of the body is duplicated, or an incomplete twin is attached to a fully developed twin)

In broad terms, conjoined twins may be regarded as a doubling anomaly. The later the incomplete embryologic separation occurs, the higher the likelihood of a complicated fusion.

Treating conjoined twins can be a daunting challenge for the surgeon. 

Successful separation of a set of omphalopagus twins was first reported in 1689 in Basel, Switzerland.[6]  Whereas series of experience with conjoined twins have been reported in the literature,[7, 8, 9]  successful separation of large numbers of conjoined twins has been limited to a few centers in the world, including the following:

  • 13 sets described by O'Neill et al from the Children's Hospital of Philadelphia in 1988 [10]
  • 14 sets reported by Mackenzie et al from the same hospital in 2002 [11]
  • 17 sets reported by Spitz and Kiely in the same year from the Great Ormond Street Hospital, London [12]
  • 46 sets reported by Rode et al from Red Cross Children's Hospital, Cape Town, South Africa [13]

Surgical experience with conjoined twins has also been reported in other parts of the world, such as China, Saudi Arabia, and New Zealand.[14, 15, 16]

In Singapore in 2003, skull and brain separation of 29-year-old craniopagus Iranian twins, Ladan and Laleh Bijani, was unsuccessful, and the sisters died of exsanguination on the operating table.

Craniopagus conjoined twins Mohamed and Ahmed Ibrahim, born in Egypt on June 2, 2001, were separated at the Children's Medical Center in Dallas, TX, on October 12, 2003, during a 34-hour operation. The operation involved skin grafting and separation of the venous sinuses. Later, they underwent skull reconstructive surgery, received rehabilitation therapy, and, finally, left for Egypt on November 19, 2005. A successfully staged separation of craniopagus twins (Carl and Clarence Aguirre) took place from 2003 to 2004 at Montefiore Medical Center in New York.

Cases of conjoined twinning often give rise to religious,[17]  moral, ethical,[18]  and legal[19, 20, 21]  concerns. For example, in England, the separation of a pair of conjoined twins from Malta, born August 8, 2000, raised considerable ethical and legal issues.[22]  The parents refused to grant permission for surgery, and the matter was referred to court. The judges concluded that separation was in the best interests of both children, even though it meant the death of the weaker twin. In the United States, the decision of the parents might have been final.

A support group for conjoined twins, Conjoined Twins International, was founded in 1996 in Prescott, Arizona, by the grandparents of a set of conjoined twins. Facebook also has a support circle of friends of conjoined twins. In addition, blog sites have been set up for certain sets of twins.


The morula becomes a blastocyst on day 6 after the ovum is fertilized. An inner cell mass develops at one end within this vesicle. The inner cell mass can form a whole fetus. Conjoined twins are produced when this inner cell mass, derived from a single zygote, incompletely splits late, after the 12th day of gestational life (see the image below).

The term conjoined twinning refers to an incomplet The term conjoined twinning refers to an incomplete splitting of monozygotic twins after 12 days of embryogenesis. Image created by Samantha Cloutier and Aaron Cloutier.

The delivered fetuses are physically joined at some point as a result. The larger the connecting bridge between the twins, the more complex its contents. Conjoined twins are classified according to their point of union is used to classify twins; the terms used include the Greek word pagos, which means "that which is fixed" (see Presentation). An especially rare condition occurs when one incompletely formed (ie, parasitic) twin is dependent on the well-formed one. This is known as heteropagus twinning. (See the images below.)

This is a much more rare occurrence termed heterop This is a much more rare occurrence termed heteropagus twinning. One twin is not fully formed (parasitic twin) and is dependent on the well-formed one. A malformed head, upper limb, and large ears are present in the parasitic twin. Three legs (tripus) are present.
Ethical issues arose in this set. The parents refu Ethical issues arose in this set. The parents refused separation on the basis that the nonviable twin was still breathing. The wishes of the parents were respected. This set of twins was lost to follow-up.

Another example of parasitic twin, in which the head is not visible, and the ethical decision is easier to make, is shown below.

An example of parasitic twin, in which the head is An example of parasitic twin, in which the head is not visible.

A nonoperated case of parasitic twins, presenting late, is shown below.

A nonoperated case of parasitic twins, presenting A nonoperated case of parasitic twins, presenting late.

The parasitic twin (sacrificed) after separation is shown below.

The parasitic twin (sacrificed) after separation. The parasitic twin (sacrificed) after separation.

Multiple limbs attached to the thorax are shown below.

Multiple limbs attached to the thorax. Multiple limbs attached to the thorax.


The incomplete anatomic separation between monozygotic twins occurs sporadically, with no increased risk in future pregnancies.

Seven cases (two published) have been reported in which conjoined twinning occurred with the use of clomiphene for induction of ovulation. Two cases of thoracopagus have been reported in which conjoined twinning occurred with periconceptional maternal griseofulvin intake. Spina bifida is associated with conjoined twinning, and one case of conjoined twinning after maternal exposure to valproic acid has been reported.

No gene mapping or linkage analysis currently exists for the malformation. Some investigators implicate abnormal X inactivation. The latter may be related to the increased incidence in female twins. Other studies refute abnormal X-chromosome inactivation.


Twinning occurs in approximately 1 of every 87 live births. Monozygotic twins account for one third of twin births. Conjoined twins account for 1% of monozygotic twins. In the United States, the incidence is 1 per 33,000-165,000 births and 1 per 200,000 live births.[23] The stillbirth rate is 40-60%. More cases are being reported now because of the routine use of fetal ultrasonography (US).

The condition is probably more common in Indian and African populations than in Whites. Exact epidemiology across different races and nations is not known because of underreporting and lack of facilities for antenatal diagnosis.

When born live, females are affected more often than males, with a female-to-male ratio of 3:1 or greater. Stillborn conjoined twins are predominantly male.

Conjoined twins show characteristic points of attachment. They are classified according to the site of union, with the following frequency:

  • Thoracoomphalopagus (ie, joined at the chest, abdomen, or both), 74% - Thoracopagus or xiphopagus (ie, joined at the chest), 40%; omphalopagus (ie, joined at the abdomen; see the first and second images below), 34%
  • Pygopagus (ie, joined at the buttocks), 18%
  • Ischiopagus (ie, joined at the ischium), 6%
  • Craniopagus (ie, joined at the head; see the third and fourth images below), 2%
Omphalopagus twins. Omphalopagus twins.
Omphalopagus twins. Omphalopagus twins.
Craniopagus twins. Craniopagus twins.
A radiograph of the skull, showing the conjoined h A radiograph of the skull, showing the conjoined heads in the case in previous two images.

Magnetic resonance imaging (MRI) or computed tomography (CT) may show the brain structure, whereas magnetic resonance angiography (MRA) can show the extent of vascular connections (see the image below).

An MRI or CT may show the brain structure, while a An MRI or CT may show the brain structure, while a magnetic resonance angiography can show the extent of vascular connections.


Several factors may predict or influence outcome, including the following:

  • Antenatal MRI and echocardiography - These techniques can be performed to accurately define the conjoined anatomy, and an outcome can be predicted, though the echocardiogram can underestimate the complexity of the cardiac anatomy; although 9 weeks is the earliest gestational age at which conjoined twins have been detected, false-positive cases are common before 10-11 weeks, as the fetal movements are limited; monoamniotic twins may appear conjoined
  • Postnatal surgery - Generally, the twins are separated at age 6-12 months, and the prognosis improves with elective surgery and meticulous preoperative planning; cardiac anatomy occasionally necessitates urgent separation at birth; however, urgent operations carry a high (40-80%) mortality
  • Skin closure material - Use of skin expanders and prosthetic mesh has improved outcome through providing better wound closure; however, use of prosthetic mesh for abdominal closure often results in fistula formation
  • Thoracopagus with cardiac conjunction - The prognosis for separating twins with conjoined ventricles remains poor, unless two cadaveric cardiac transplants were made available; only one case of successful separation of conjoined atria has been reported; factors associated with increased survival include elective separation, older age, and cardiac conjunction involving only shared pericardium [24]
  • Craniopagus - Usually, the twins have two largely separate brains; however, the venous sinus may be shared, and division of this sinus carries a high risk for life-threatening exsanguination


History and Physical Examination

Cephalothoracopagus twinning is characterized by the anterior union of the upper half of the body, with two faces angulated variably on a conjoined head. The anomaly is occasionally known as janiceps, named after the two-faced Roman god Janus. The prognosis is extremely poor because surgical separation is not an option, in that only a single brain and a single heart are present and the gastrointestinal (GI) tracts are fused. This malformation is extremely rare (see the image below).

Cephalothoracopagus twins. Cephalothoracopagus twins.

Craniopagus occurs in 2% of conjoined twins. In this variety, the twins have cranial fusion (see the images below). Stone and Goodrich subclassified craniopagus into four varieties, depending on whether a significantly shared dural venous sinus system (total vs partial) is present and on whether the intertwin longitudinal angulation is below 140º.[25]

Craniopagus twins. Craniopagus twins.
Craniopagus twins. Craniopagus twins.

Bicephalus or dicephalus means that two heads are present on a single trunk (see the images below).

This set of conjoined twins was a stillbirth. Pren This set of conjoined twins was a stillbirth. Prenatal ultrasonography failed to reveal the second head. An emergent cesarean section was performed after failure to progress.
This is an example of double-headed (bicephalus) c This is an example of double-headed (bicephalus) conjoined twins, a rare occurrence.

In omphalopagus, the anterior abdomens are united (see the image below). Omphalopagus (34%) is considered a subset of thoracoomphalopagus. Its incidence is usually added to that of thoracopagus (40%) to yield the overall incidence of thoracoomphalopagus (74%). Pure omphalopagus twins have no cardiac union.

Thoracoomphalopagus twins. Thoracoomphalopagus twins.

A case of thoracoomphalopagus twins, after separation, is shown below.

A case of thoracoomphalopagus twins, after separat A case of thoracoomphalopagus twins, after separation.

In the condition parapagus, (see the image below) or diprosopus,[26]  twins have lateral union of their trunks so that both faces are forward looking in the same plane.

Parapagus twins. Parapagus twins.

Pygopagus (see the image below) is the term used when the twins face in opposite directions. The sacra are fused, and the twins may share a portion of the spinal cord. In addition, the rectum and perineal structures are usually fused.

Pygopagus twins. Pygopagus twins.

In rachipagus (see the image below), the twins are joined back-to-back at any point, usually above the lumbar spine. They may have extensive vertebral fusion in the dorsal midline and may have meningocele, neural connection, or both. One of the embryos generally fails to survive, leaving some body part attached to the other twin's dorsal column. In these cases, rachipagus must be differentiated from fetus in feto (within a body cavity) and teratoma (no mature organ).[27]

Rachipagus twins. Rachipagus twins.

Thoracopagus is the most common variety, occurring in 40% of conjoined twins. The chests are joined, and the hearts are almost always fused in some way (see the images below). As noted above, thoracopagus is often combined with omphalopagus.

Conjoined twins unified at the thorax and abdomen. Conjoined twins unified at the thorax and abdomen.
Another view of the same set of twins as shown in Another view of the same set of twins as shown in the previous image.
A superior radiographic view of the same set of tw A superior radiographic view of the same set of twins as shown in the previous 2 images. The twins have 2 hearts that are not conjoined, making this a possible operation.
Esophagram showing union at the level of stomach. Esophagram showing union at the level of stomach.
Thoracoomphalopagus twins. Thoracoomphalopagus twins.
Thoracoomphalopagus twins. Thoracoomphalopagus twins.

In ischiopagus twins (see the image below), the lower abdomen and the pelves are fused.[28, 29]  The twins may have three legs (ie, tripus) or four (ie, tetrapus). The genitourinary system and the rectum are shared; the liver may also be fused.

Ischiopagus tetrapus twins. Ischiopagus tetrapus twins.


Laboratory Studies

Chromosomal studies are inconclusive. An abnormal X-chromosome inactivation has been proposed, but this has not been proved.

Amniocentesis with an estimation of the lecithin-sphingomyelin ratio is performed to assess fetal lung maturity and to determine the optimal time for a cesarean delivery.

Imaging Studies

Antenatal ultrasonography (US) can reveal conjoined twinning as early as 8 weeks' gestation.[30] Conversely, twins with extreme fusion may be mistaken for a singleton. The twin fetuses do not move apart with fetal movement. Polyhydramnios is frequent (75%). A monoamniotic cavity is present, and more than three umbilical vessels may be observed. Fusion sites include the following:

  • Thorax (thoracopagus)
  • Abdomen (omphalopagus)
  • Pelvis (ischiopagus)
  • Sacrum (pygopagus)
  • Skull (craniopagus)

Extensive zones of fusion may be named by applying the prefix di- (meaning two), followed by the portion of the twins that is unfused. Examples include dicephalus (two heads on one body) and dipygus (double buttocks, with a single head, single torso, separate pelves, and four legs). Serial scans may be required to monitor for hydrops.

Antenatal echocardiography has better yield than postnatal echocardiography in thoracopagus twins, because surface scanning may be difficult.

Magnetic resonance imaging (MRI) of the brain is performed, along with magnetic resonance angiography (MRA) and magnetic resonance venography (MRV) to delineate structures and blood supply in the craniopagus variety.

Radionuclide angiography is performed to calculate the extent of cross-circulation. Selective angiography is usually not necessary in twins with a shared liver.

US allows for a complete anatomic examination and a thorough search for associated lethal malformations. A detailed ultrasound examination to exclude the possibility of conjoined twins is mandatory in all multiple pregnancies. Two-dimensional (2D) US is instrumental in diagnosing conjoined twins antenatally, but precise classification is difficult because of three-dimensional (3D) structures. 3D US has shown promise in improving the visualization of complex anatomic spatial relations.

Abdominal US is performed to determine how many gallbladders are present (ie, one or two) and to determine the polarity of the liver and pancreas if these organs are also conjoined. Often, however, this determination may not be precise.

Advantages of US include the following:

  • Images other structures (eg, aorta, pancreas, liver)
  • Identifies complications (eg, stenosis, obstruction)
  • Can be rapidly performed at the bedside
  • Does not involve radiation (important in pregnancy)

Disadvantages of US include the following:

  • Depends on the type and extent of fusion, as well as on the operator's abilities
  • Cannot image the ductal system proximal to the common bile duct
  • Has less sensitivity for the site and extent of duplication or fusion as compared with computed tomography (CT) or MRI

Fetal MRI can identify shared anatomy of twins with precise detail.[30] However, this test is not 100% accurate.

Although CT has been used in some studies,[31] its yield for complex anatomy is lower than that of MRI. CT may be helpful in cases of shared bony pelvis and shared pelvic perineal muscles.

Contrast studies are performed to evaluate the extent of gastrointestinal (see the image below), genitourinary, and reproductive system fusions.

Esophagram showing union at the level of stomach. Esophagram showing union at the level of stomach.

Diisopropyl iminodiacetic acid (DISIDA) scanning is a nuclear medicine study performed to visualize the biliary tract. A technetium-labeled analogue of DISIDA is administered intravenously (IV) and is secreted by hepatocytes into bile, enabling visualization of the gallbladder and biliary tree in 30 minutes.

Other Tests

On electrocardiography (ECG), a single QRS indicates that cardiac separation is not possible. However, the presence of two separate patterns does not guarantee a successful separation.

Electroencephalography (EEG) may be performed to evaluate baseline brain activity in craniopagus twins.


Cardiac catheterization is performed to determine the nature of complex cardiac anomalies. An accurate estimation of all major inflow and outflow vessels should be made.



Approach Considerations

Historically, conjoined twins in general have been placed into three groups:

  • Group 1 - Those who do not survive delivery plus those who die shortly after birth
  • Group 2 - Those who survive to undergo an elective procedure
  • Group 3 - Those in whom an emergency procedure is required

Of all types of conjoined twins, omphalopagus twins are the most favorable candidates for elective surgery because of good survival rates.[32]  A conjoined heart is a contraindication for surgery because the heart complex is usually inseparable; postoperatively, the divided heart often goes into congestive cardiac failure. Cephalopagus twins with extensive brain union cannot be separated. Furthermore, in the United States, parental refusal is a contraindication for surgery.

Emergency conditions may arise at any time; examples include intestinal obstruction, rupture of an omphalocele, congestive cardiac failure, severe degree of respiratory compromise, and terminal illness in one of the twins.

Harper and Kenigsberg suggested that the abdominal cavities grow as the twins age, whereas the bridge connecting the omphalopagus twins does not grow in diameter.[33]  Hence, abdominal wall closure can be performed more easily at approximately age 1 year.

Ethical,[34] legal,[21]  and religious questions frequently arise with conjoined twins. For example, will the surgery be successful? Is sacrificing one twin to save the other justified? The moral and ethical aspects of separation must be considered, especially in the following circumstances:

  • A choice must be made concerning single organ systems - The twin who receives the organ system will live and thrive, whereas the other twin will suffer or die; a similar problem arises when unequal limbs are present
  • The twins have conjoined hearts - Surgical separation of the cardiac complex has been mostly unsuccessful; in some cases, one twin is allowed to live with the entire cardiac complex; the availability of two heart transplants at the same time may improve the options
  • The twins are craniopagus and have complete brain junction - These twins are usually inseparable

Sometimes, the choice is made to sacrifice one twin in order to give the other one a chance of survival. In these instances, consulting with the hospital ethics committee before going to the courts has helped. In these cases, a legal ruling must be made before surgery can be performed.

Medical Therapy

Nutritional support is very important for the survival of the separated twins, in that delayed healing and infections may occur. Parenteral or enteral feedings must be carefully planned.

Surgical Therapy

Once the diagnosis has been confirmed, the parents should be counseled on the possible outcomes. The team that will attempt the possible separation should provide this advice. The delivery should take place close to the surgical unit where the separation will be performed. With antenatal diagnosis, the delivery should always be by cesarean section. An elective cesarean delivery should be performed near term after confirmation of fetal lung maturity. Twin delivery may otherwise lead to overdistention and uterine atony.

The following are indications for emergency separation:

  • One or both twins are in a life-threatening situation
  • A correctable life-threatening associated congenital anomaly (eg, intestinal atresia, malrotation with midgut volvulus, ruptured exomphalos, or anorectal agenesis) is present

If the condition of the twins is stable and early separation is not indicated, surgery is usually delayed until age 6-12 months. At that age, the twins are larger and better able to tolerate the surgical procedure. Rapid expansion of the body wall can occur to close substantial defects. The anatomy of the junction and the shared organs and structures will dictate the technical details of the procedure.

Reconstructive surgery is important in ischiopagus twins, who require gastrointestinal (GI), genitourinary (GU), reproductive, or skeletal reconstruction. One twin keeps the shared anus and rectal canal, and the other receives reconstructed ones. Reconstruction is easier in twins with tetrapus than in twins with tripus.

Preparation for surgery

A systematic approach to the workup is necessary. The type of conjoined twinning present determines the specific studies needed; the possible areas of fusion are predictable in each type. Three-dimensional (3D) models[35, 36]  should be built to depict the shared anatomy.[37]  The shared structures must be divided, if possible, in such a way as to maintain functional integrity for each twin. Some shared organs (eg, a single rectum) may not be divisible. Particular considerations for different organ systems are as follows:

  • Cardiovascular system - Antenatal and postnatal echocardiography must be performed to accurately define the extracardiac and intracardiac anomalies, and catheterization should be performed in complex cardiac anomalies; electrocardiography (ECG) and radionucleotide angiography may be helpful in deciding whether a separation operation can be carried out; a single QRS complex is an ominous sign
  • Hepatobiliary tree - The number of gallbladders, livers, and pancreata should be known; the conjoined liver may be oriented in an oblique plane to the axis of the twins; hepatic venous drainage for each twin must be ascertained; a portoenterostomy or even a cutaneous biliary fistula in one of the separated twins may be required if only a single extrahepatic biliary tree exists
  • GI tract - This should be evaluated from top to bottom because fusion may occur at any point; an anorectal malformation (imperforate anus) may be present; laparoscopy may be used to determine bowel distribution between the twins [38]
  • Urinary tract - Imaging studies are performed to assess the status (eg, number, reflux, anatomy) of the kidneys, ureters, urinary bladders, and urethras
  • Genital system - In pygopagus and ischiopagus twins, the number and duplicity of vaginas and the presence of a cervix and urogenital sinus must be noted; in males, penile and scrotal status must be established
  • Central nervous system - The nature and extent of nervous system junction must be established in pygopagus twins, who may also have associated hemivertebrae
  • Skin - Vascular territories of the skin must be mapped with intravenous (IV) fluorescein dye; the amount of skin needed to close the wound after separation must be evaluated; skin and tissue expanders may be required to allow subsequent wound closure
  • Musculoskeletal system - All postoperative defects of the muscle and bone must be covered

Preoperative team conferences should be held, reviewing all the details and models. Moral, ethical, and legal issues must be fully addressed before surgery.

Operative details

Two complete anesthesia teams are required. Two surgical teams are necessary after the twins are apart. Full monitoring is necessary. All administered drugs and IV fluids are calculated on a total-weight basis, with half being delivered to each twin. The IV drugs may have an unpredictable effect because of the cross-circulation. Thus, particular care is needed when these drugs are administered.

During the procedure, abnormal vascular communications and previously unidentified anomalies (eg, GI and GU malformations) may be encountered. Unexpected findings are common even after an extensive preoperative evaluation. The surgical team should be aware of this and should be prepared to vary the operative procedure accordingly.

Where primary skin closure of the defect would otherwise be impossible, Silastic skin expanders have been used to achieve tension-free closure. A tight thoracic wall closure can lead to cardiac tamponade. Absorbable synthetic mesh has been used with success to close thoracic and abdominal gaps. A porous polyethylene implant and a monofilament polypropylene patch have been suggested for reconstruction of the sternum and abdominal wall, respectively.[39]

A skin graft from a nonviable twin may be used in certain cases. Because skin grafts need intact body wall for a bed, they are not a very useful primary form of skin coverage.[6]

Postoperative Care

Monitoring must be continued postoperatively in the intensive care unit (ICU). Because surgery is prolonged, infants are mechanically ventilated for a variable period. Fluid and electrolyte balance should be closely monitored. Sepsis is a major cause of morbidity and mortality, and precautions must be exercised, particularly when large skin defects are present.


Complications are common and may include the following:

  • Congestive cardiac failure - This is observed when a conjoined heart is divided
  • Inadequate or incomplete organ systems - This complication occurs when the twins have unequal distribution of their organs (eg, one shared biliary tract); adequate preplanning in these cases is essential
  • Enormous skin defect - This may result from separation of a large bridge and can often be avoided by delaying the surgery until age 1 year; preseparation tissue expanders (essentially pouches that are gradually filled with saline solution) may be useful for avoiding this problem because by stretching the skin, they allow the surgeons to close the wounds after separation surgery
  • Infection - Strict infectious precautions are required for 2 days after the surgical procedure, and patients are kept in the ICU; if they do well, routine precautions are adequate for follow-up care
  • Hemorrhage - Life-threatening exsanguination can result, especially in craniopagus twins who have a large communicating venous sinus