"'Conjoined twins" are identical twins joined in utero. An extremely rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence in Southeast Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.

Two contradicting theories exist to explain the origins of conjoined twins. The more generally accepted theory is "fission", in which the fertilized egg splits partially. The other theory, no longer believed to be the basis of conjoined twinning, is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to conjoined twins as there are some monozygotic but non-conjoined twins who also share these structures in utero.

The most famous pair of conjoined twins was Chang and Eng Bunker (Thai: อิน-จัน, In-Chan) (1811–1874), Thai brothers born in Siam, now Thailand. They traveled with P.T. Barnum's circus for many years and were labeled as the Siamese twins. Chang and Eng were joined at the torso by a band of flesh, cartilage, and their fused livers. In modern times, they could have been easily separated. Due to the brothers' fame and the rarity of the condition, the term "Siamese twins" came to be used as a synonym for conjoined twins.

Surgery to separate conjoined twins may range from very easy to very difficult depending on the point of attachment and the internal parts that are shared. Most cases of separation are extremely risky and life-threatening. In many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head or share a vital organ. This makes the ethics of surgical separation, where the twins can survive if not separated, contentious. Alice Dreger of Northwestern University found the quality of life of twins who remain conjoined to be higher than is commonly supposed. Lori and George Schappell and Abby and Brittany Hensel are notable examples.

In 1955, neurosurgeon Harold Voris (1902-1980) and his team at Mercy Hospital in Chicago performed the first successful operation to separate Siamese twins conjoined (Craniopagus twins) at the head, which resulted in long-term survival for both. The larger girl was reported in 1963 as developing normally, but the smaller was permanently impaired.

In 1957, Bertram Katz and his surgical team made international medical history performing the world's first successful separation of conjoined twins sharing a vital organ. Omphalopagus twins John Nelson and James Edward Freeman (Johnny and Jimmy) were born to Mr. and Mrs. William Freeman of Youngstown, Ohio, on April 27, 1956. The boys shared a liver but had separate hearts and were successfully separated at North Side Hospital in Youngstown, Ohio by Bertram Katz. The operation was funded by the Ohio Crippled Children's Service Society.

Recent successful separations of conjoined twins include that of the separation of Ganga & Jamuna Shreshta in 2001, who were born in Kathmandu, Nepal, in 2000. The 197-hour surgery on the pair of craniopagus twins was a landmark one which took place in Singapore; the team was led by neurosurgeons Chumpon Chan and Keith Goh. The surgery left Ganga with brain damage and Jamuna unable to walk. Seven years later, Ganga Shrestha died at the Model Hospital in Kathmandu in July 2009, at the age of 8, three days after being admitted for treatment of a severe chest infection.

A case of particular interest was that of infants Rose and Grace ("Mary" and "Jodie") Attard, conjoined twins from Malta who were separated in Great Britain by court order over the religious objections of their parents, Michaelangelo and Rina Attard. The surgery took place in November, 2000, at St Mary's Hospital in Manchester. The operation was controversial because Rose, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Grace's. (The twins were attached at the lower abdomen and spine.) However, if the operation had not taken place, it was certain that both twins would die. Grace survived to enjoy a normal childhood.

In 2003 two 29-year-old women from Iran, Ladan and Laleh Bijani, who were joined at the head but had separate brains (craniopagus) were surgically separated in Singapore, despite surgeons' warnings that the operation could be fatal to one or both. Their complex case was accepted only because high tech graphical imagery and modelling would allow the medical team to plan the risky surgery. Unfortunately, an undetected major vein hidden from the scans was discovered during the operation. The separation was completed but both women died while still in surgery on July 8, 2003.

With so few individuals actually surviving until birth, the only treatment option is surgery to try to remove the parasitic twin. Surgery, however, is very dangerous and has been successful only once. The problem with surgical intervention is that the arterial supplies of the head are so intertwined that it is very hard to control the bleeding, and it has been suggested that cutting off the parasitic twin's arterial supply might improve the odds of the developed twin's survival.

For every 200,000 live births, conjoined twins are born. Conjoined twins are identical and of the same sex one hundred percent of the time and are more common in females than in males. For surgical separations the survival rate of at least one twin surviving is approximately 75%.

Two-headed people and animals, though rare, have long been known to exist and documented.

Polycephaly is the condition of having more than one head. The term is derived from the Egyptian stems "poly" (Greek: "πολύ") meaning "many" and "kephalē" (Greek: "κεφάλη") meaning "head". A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.

Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.

In humans, there are two forms of twinning that can lead to two heads being supported by a single torso. In dicephalus parapagus dipus, the two heads are side by side. In craniopagus parasiticus, the two heads are joined directly to each other, but only one head has a functional torso. Survival to adulthood is rare, but does occur in some forms of dicephalus parapagus dipus.

There are many occurrences of multi-headed animals in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.

Craniopagus twins are conjoined twins that are fused at the cranium. This condition occurs in about 10–20 babies in every million births in the United States. Among this small group, cephalic conjoining, or craniopagus twinning, represents the rarest of congenital abnormalities, accounting for 2–6% of all conjoined twins. Additionally, conjoined twins are genetically identical and always share the same sex. The union in craniopagus twins may occur on any portion of the Calvary, but does not include either the face or the foramen magnum. The thorax and abdomen are separate and each twin has its own umbilicus and umbilical cord. The union may involve the entire diameter of the head or only a small portion. This suggests that although there are many different kinds of vulnerabilities already known in the scientific community, there are an infinite number of variations that can occur. Most of these variations are based on the rotation of one twin's skull to the other and the different phenotype sub-groups of craniopagus twins are based on all these rotational conformations. Each of these factors (rotation, spot of union) affects the development of the brain, the vascular system within the brain and overall wellness of life both of the twins have outside the womb. Relatively few craniopagus twins survive the perinatal period – approximately 40% of conjoined twins are stillborn and an additional 33% die within the immediate perinatal period, usually from organ abnormalities and failure. However 25% of craniopagus twins survive and can be considered for a surgical separation and several attempts occur yearly worldwide. In the last-half century, many advances in medicine including brain imaging, neuro-anesthesia and neurosurgical techniques have proven that a successful outcome is possible following separation of total craniopagus twins.

The girls underwent a six-week period for physical therapy and recovery. Maria and Teresa are now able to walk independently and are starting to form their own individual personalities. The girls were able to return to the Dominican Republic and often return to the United States for follow-up care.

Clarence and Carl Aguirre (born April 21, 2002) are former conjoined twins born in Silay City, Philippines. They were conjoined at the top of the head (vertical craniopagus). In 2003 they were brought to the United States by their mother Arlene in the hope of having them surgically separated. A revolutionary new process was used in separating the twin boys. Because "marathon" surgical operations have historically led to a high rate of mortality and morbidity, Clarence and Carl's doctors chose to separate them in several smaller operations, allowing the twins to recuperate after each surgery.

In October 2003, tissue expanders (pouches filled with saline) were implanted under the boys' scalps. This is common practice in separating conjoined twins, as it creates more skin that can be used to cover the twins' wounds after separation. Over the next ten months, the brothers underwent several more operations to tease apart their joined skulls and brains. On August 4, 2004, the final surgery was completed. All of these operations were performed at Montefiore Medical Center in the Bronx, New York by a team led by Dr. James T. Goodrich, a pediatric neurosurgeon and Dr. David A. Staffenberg, a plastic surgeon. They received most of their post-operative rehabilitation at Blythedale Children's Hospital in Valhalla, New York (Westchester County).

By December 2005, Clarence could walk short distances unassisted, and Carl could walk with a walker. As of August 2014 both boys and their mother live in a donated house in Scarsdale, New York. The twins continue to attend physical therapy at Blythedale. Goodrich says there was some degeneration of Carl’s right parietal lobe: he takes medication to control seizures, can utter just a word or two at a time, and has limited use of his left arm and leg. Both must still wear helmets to protect their brains; when they are fully grown, their skulls will be patched.

In the past, the use of terminology when describing parasitic twins has been somewhat inconsistent. By definition, a parasitic twin is joined to another twin in a certain anatomical location or position on the developed twin's body. The underdeveloped twin is termed the parasite, and the developed twin is termed the autosite. The autosite can have some abnormalities, as well. For the most part, however, it has developed enough that it can live on its own.

Joseph Banda and Luka Banda (born January 23, 1997, in Lusaka, Zambia) are twin brothers who used to be conjoined. They were born joined at the back of the skull and faced in opposite directions. In 1997, Dr. Ben Carson led a team of 50 Zambian and South African specialists to separate the 11-month-old twins in a 22-hour operation. They did not share any organs, but shared intricate blood vessels that flowed into each other's brains. In 2001, the twins were fitted with artificial skulls to permanently close their heads, and are neurologically normal. In an interview, Dr. Carson stated about the operation:

They are residing in a low-cost/high-density area of Lusaka called Chilenje South.

Kendra Deene Herrin and Maliyah Mae Herrin (born February 26, 2002). are former conjoined twins. They were separated in August 2006. They were the first set of conjoined twins to be separated to share a kidney. Kendra retained their shared kidney following the separation surgery, while Maliyah Herrin underwent dialysis until she was transplanted with a kidney donated by her mother in April 2007.

Prior to the separation surgery, medical ethicists in the United States and in Europe discussed whether it was ethical to separate the twins because of the additional risk to Maliyah. Their family and doctors believed separation surgery would give both girls their best chance to live a normal life.

Krista and Tatiana Hogan (born October 25, 2006) are Canadians who are conjoined craniopagus twins. They are joined at the head (the top, back, and sides). They were born in Vancouver, British Columbia and are the only unseparated ones of that type currently alive in Canada. They live with their mother, Felicia Simms, in Vernon, British Columbia and often travel to Vancouver for care at BC Children's Hospital and Sunny Hill Health Centre for Children.

Following their separation surgery, both girls began learning to walk using crutches. Their parents have discussed having them eventually fitted with prosthetic legs. They also get around using wheelchairs or by crawling and pulling themselves up on furniture, according to the family's website. Jake and Erin Herrin's other children include fraternal twin sons and another daughter.

Sirenomelia, alternatively known as Mermaid syndrome, is a rare congenital deformity in which the legs are fused together, giving them the appearance of a mermaid's tail as the nickname suggests.

This condition is found in approximately one out of every 100,000 live births (about as rare as conjoined twins) and is usually fatal within a day or two of birth because of complications associated with abnormal kidney and urinary bladder development and function. More than half the cases of sirenomelia result in stillbirth and this condition is 100 times more likely to occur in identical twins than in single births or fraternal twins. It results from a failure of normal vascular supply from the lower aorta in utero. Maternal diabetes has been associated with caudal regression syndrome and sirenomelia, although a few sources question this association.

VACTERL-H is an expanded form of the VACTERL association that concludes that this diagnosis is a less severe form of sirenomelia. The disorder was formerly thought to be an extreme case of caudal regression syndrome; however, it was reclassified to be considered a separate condition.

Based on statistics, the twins were given a 20% chance of survival at birth . At birth at B.C. Women's Hospital & Health Centre, they were described as "wriggly, vigorous and very vocal." They weighed twelve and a half pounds, not six and a half pounds as reported by some media outlets, when they were born by caesarean section.

Ganga and Jamuna Mondal (born Ayara and Jayara Ratun, 1969 or 1970), known professionally as The Spider Girls and The Spider Sisters, are conjoined twins from a Bengali family in Basirhat, West Bengal, India.

Anastasia and Tatiana Dogaru

born August 29, 2004) are craniopagus conjoined twins. They were scheduled to begin the first of several surgeries to separate them at Rainbow Babies and Children's Medical Center in Cleveland, Ohio. However, in August 2007 the surgery was called off as too dangerous.

The twins were born in Rome, Italy to Romanian parents, Alin Dogaru, a Byzantine Catholic priest, and Claudia Dogaru, a nurse. Their mother heard about the successful separation of Egyptian-born twins who were also joined at the head and hoped her children could also be successfully separated. The Dogaru family — who also have an older daughter, Maria, and younger son Theodor — were brought to north Texas by the World Craniofacial Foundation to have Anastasia and Tatiana evaluated for possible separation.

The girls are currently developing normally for their age and speak both Romanian and English. They get around with Anastasia leading the way and Tatiana following. The top of Tatiana's head is attached to the back of Anastasia's. Anastasia, whose kidneys don't function, relies on her sister's kidneys, and Tatiana on her sister's circulatory system. The girls also share blood flow to the back of the brain and some brain matter. Doctors estimated the twins had only a 50 percent chance of surviving the surgery. There were also risks of complications, such as brain damage, but the girls also risk early death if they remain conjoined. Their parents believed separation would give them their best chance at living a normal life.

In May 2007, doctors used a catheter to insert wire coils into the veins of the two girls, successfully redirecting their blood flow. It was the first time the procedure was attempted in conjoined twins. Doctors pushed back the first of the planned separation surgeries to June 2007 while studying the complex circulatory system of the twins, but, in August of that year, decided it was too risky.

Diprosopus (Greek , "two-faced", from , ', "two" and , ' [neuter], "face", "person"; with Latin ending), also known as craniofacial duplication (cranio- from Greek , "skull", the other parts Latin), is an extremely rare congenital disorder whereby parts (accessories) or all of the face are duplicated on the head.

Only a few individuals who did not have fatal kidney and bladder complications are known to have survived beyond birth with this condition.

TRAP sequence can be diagnosed using obstetric ultrasound. Doppler interrogation will confirm that blood flow in the acardiac twin is in the reverse direction, entering via the umbilical cord artery and exiting through the vein.

A parasitic twin (also known as an asymmetrical or unequal conjoined twin) is the result of the processes that also produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.

The independent twin is called the autosite.

Giacomo and Giovanni Battista Tocci were two-headed conjoined twins born in Locana, Italy between 1875 and 1877 on either July or October 4.

Syndactyly of the border digits (thumb/index finger or ring/small fingers) is treated at early age to prevent the larger digit from curving towards the smaller digit with growth. Typically, syndactyly of these digits is treated at six months of age. The treatment of syndactyly of the other digits is elective and is more commonly performed when the digits have grown, at 18–24 months of age.

In 1962, J. Selwyn Crawford MD from the British Research Council defined a nuchal cord as one that is wrapped 360 degrees around the fetal neck. Dr. Crawford commented "It is all the more remarkable, therefore, that little work has been done ... to analyze its effects during labor and delivery". To date, there is no prospective case control double-blind study looking at nuchal cords and observational studies vary in opinion as to the degree of poor outcomes. Also not included in these studies is which umbilical cord form (of the 8 different possible structures) was considered a nuchal cord.

Ultrasound diagnosis of a cord around the neck was first described in 1982. “Coils occur in about 25% of cases and ordinarily do no harm, but occasionally they may be so tight that constriction of the umbilical vessels and consequent hypoxia result.” Williams Obstetrics 16th Edition, has only one single sentence in the entire textbook regarding cords around the neck. By contrast, the First Edition of the Encyclopædia Britannica from 1770 had 20 pages of information about Umbilical Cord Pathology with drawings of Umbilical Cord Entanglement. The Royal College of Obstetricians and Gynaecologists has these images on its brochure. There are currently three recent texts on ultrasonography which demonstrate the ability of ultrasound to identify umbilical cord issues with reliability as of 2009.

A study published in 2004 was done to establish the sensitivity of ultrasound in the diagnosis of a nuchal cord. Each of 289 women, induced the same day, underwent a transabdominal ultrasound scan with an Aloka 1700 ultrasound machine with a 3.5 MHz abdominal probe, using gray-scale and color Doppler imaging immediately prior to induction of labor. Presence of the cord was sought in the transverse and sagittal plane of the neck. A nuchal cord was diagnosed if the cord was visualized lying around at least 3 of the 4 sides of the neck. A cord was actually present at delivery in 52 of the 289 women. Only 18 of the 52 cords or 35% of the nuchal cords were detected on ultrasound done immediately before delivery, and 65% of nuchal cords were not detected. Of the 237 cases where there was no cord at delivery, ultrasound had false positive results, i.e. diagnosed a cord in 44 of the 237 cases (19%) in which there was no cord present at all. In this study, ultrasound was only 35% accurate at finding a single loop, and only 60% accurate at detecting a nuchal cord wrapped multiple times around the neck.

In no study was it possible by ultrasound to distinguish between a loose or a tight cord, although at least 3 attempted to do so. Peregrine concludes that ultrasound diagnosis of nuchal cords will only be useful if doctors are able to do so reliably and predict which of those fetuses are likely to have a problem., However, perinatologists routinely look for umbilical cord issues in monoamniotic twins. Studies have shown an improvement in outcomes where cord entanglement was prenatally identified in these cases. Ultrasound measurement of the velocity of flow in the cord may be useful in the management of twins and chronically growth-retarded fetuses. Of course this depends on the training of the sonographer. To date there are no ultrasound courses which teach the identification of nuchal cord to physicians or technicians. A recent review by Wilson of the American Academy of Ultrasonography Technicians recommends the documentation of umbilical cord issues.

Clapp, et al, attempted to find out at what rate nuchal cords come and go during pregnancy. He recruited 84 healthy, non-smoking, non-substance abusing women carrying a single fetus, with certain dates before the 20th week of gestation. They all agreed to 4 extensive ultrasounds at 24-26, 30-32, 36–38 weeks gestation and during labor and delivery. They evaluated fetal biometry, fetal tone, fetal motion including breathing movements, amniotic fluid volume, fetal flow redistribution, velocity flow profiles from the umbilical artery at the body wall and placental insertion and at the origin of fetal middle cerebral artery. Finally, color flow Doppler imaging was used to determine whether a nuchal cord was present. Clapp reports that in 60%, or 50 of the 84 fetuses studied, a nuchal cord was seen on ultrasound at one of the 4 evaluations. He found that the presence of a nuchal cord linearly increases as the pregnancy continues. Larson, et al, found this to be true. She found, of the 13,895 singleton deliveries, a nuchal cord linearly increased every week of gestation, appearing in 6% at 20 weeks to 29.0% at 42 weeks gestation.